Add tests for CurlDownloadStrategy#tarball_path

Sometimes, `brew cask fetch`/`install` fails with an error message
similar to this:

```
==> Downloading https://w3g3a5v6.ssl.hwcdn.net/upload2/game/214692/735
Error: Download failed on Cask 'steamed-hams' with message: Operation
not supported @ rb_sysopen -
/Users/claudia/Documents/dev/brew/var/homebrew/locks/steamed-hams--1.0
.com&Expires=1520937180&Signature=CGmDulxL8pmutKTlCleNTUY%2FyO9Xyl5u9y
VZUE0uWrjadjuz67Jp7zx3H7NEOhSyOhu8nzicEHRBjr3uSoOJzwkLC8LBLKnz%2B2X%2B
iq5m6IdwSVFcLp2Q1Hr2kR7ETn3rF1DIq5o0lHCyzMmyNe5giEKJNW8WF0KXriULhzLTWL
SA3ZTLCIofAdRiiGje1kNYY3C0SBqymQB8CG3ONn5kj7CIGbxrDOq5xI2ZSJdIyPysSX7S
LvEDBw2KdR24q9t1wfjS9LUzelf5TWk6ojj8p9%2FHjl%2Fi%2FVCXNN4o1mW%2FMayy2t
TY1qcC%2FTmqI1ulZS8SNuaSgr9Iys9oDF1%2BPK%2B4Sg==&hwexp=1520937440&hwsi
g=55bc66884b925ef22f8673c33bfcc33b.incomplete.lock
```

To reproduce the issue, check out this branch and run the
`cask/download_strategy` test suite:

```
brew tests --only=cask/download_strategy
```

Or take a real-life example, which would be a Cask whose URL has
**1.** no `.` character anywhere in the URL path itself (outside of
the domain name), **and 2.** at least one query parameter with a `.`
character in it; **and 3.** other query parameters following that
with a combined length of more than 255 characters.

This combination may be uncommon but it exists, especially in
[URLs that change on every visit](1002d41242/doc/cask_language_reference/stanzas/url.md (urls-that-change-on-every-visit)),
for example
[`steamed-hams`](9d7df499cd/Casks/steamed-hams.rb)
from my `claui/cask-games` tap:

    $ brew tap claui/cask-games

    $ brew cask fetch steamed-hams

In a nutshell, **URL query strings sometimes look like a very long
file extension to Homebrew,** which it then proceeds to use as a file
name.

In `CurlDownloadStrategy`, Homebrew seems to apply a heuristic to the
cask URL in order to figure out a possibly meaningful file extension.
Sometimes this heuristic produces immensely long file extensions,
especially when there’s a query parameter with a `.` character in it
but not in the URL path itself (outside of the domain name).

Homebrew then believes that everything after the `.` is a file
extension. In one of the later steps, it tries to create a lock file
containing that extension, which fails because HFS+ cannot handle
files whose base file name has more than 255 characters.

One solution would be to improve Homebrew’s extension detector a bit
so that it won’t cross individual URL query param boundaries any
longer. This is done by adding `&` as a stop character:

```
def ext
  Pathname.new(@url).extname[/[^?&]+/]
end
```

This appears to fix the issue for most (if not all) practical
purposes.

Library/Homebrew/test/cask/download_strategy_spec.rb

@@ -93,6 +93,66 @@ describe "download strategies", :cask do
         expect(curl_args.each_cons(2)).to include(["-e", "http://somehost/also"])
       end
     end
+
+    context "with a file name trailing the URL path" do
+      describe "#tarball_path" do
+        subject { downloader.tarball_path }
+        its(:extname) { is_expected.to eq(".dmg") }
+      end
+    end
+
+    context "with no discernible file name in it" do
+      let(:url) { "http://example.com/download" }
+
+      describe "#tarball_path" do
+        subject { downloader.tarball_path }
+        its(:to_path) { is_expected.to end_with("some-cask--1.2.3.4") }
+      end
+    end
+
+    context "with a file name trailing the first query parameter" do
+      let(:url) { "http://example.com/download?file=cask.zip&a=1" }
+
+      describe "#tarball_path" do
+        subject { downloader.tarball_path }
+        its(:extname) { is_expected.to eq(".zip") }
+      end
+    end
+
+    context "with a file name trailing the second query parameter" do
+      let(:url) { "http://example.com/dl?a=1&file=cask.zip&b=2" }
+
+      describe "#tarball_path" do
+        subject { downloader.tarball_path }
+        its(:extname) { is_expected.to eq(".zip") }
+      end
+    end
+
+    context "with an unusually long query string" do
+      let(:url) do
+        [
+          "https://node49152.ssl.fancycdn.example.com",
+          "/fancycdn/node/49152/file/upload/download",
+          "?cask_class=zf920df",
+          "&cask_group=2348779087242312",
+          "&cask_archive_file_name=cask.zip",
+          "&signature=CGmDulxL8pmutKTlCleNTUY%2FyO9Xyl5u9yVZUE0",
+          "uWrjadjuz67Jp7zx3H7NEOhSyOhu8nzicEHRBjr3uSoOJzwkLC8L",
+          "BLKnz%2B2X%2Biq5m6IdwSVFcLp2Q1Hr2kR7ETn3rF1DIq5o0lHC",
+          "yzMmyNe5giEKJNW8WF0KXriULhzLTWLSA3ZTLCIofAdRiiGje1kN",
+          "YY3C0SBqymQB8CG3ONn5kj7CIGbxrDOq5xI2ZSJdIyPysSX7SLvE",
+          "DBw2KdR24q9t1wfjS9LUzelf5TWk6ojj8p9%2FHjl%2Fi%2FVCXN",
+          "N4o1mW%2FMayy2tTY1qcC%2FTmqI1ulZS8SNuaSgr9Iys9oDF1%2",
+          "BPK%2B4Sg==",
+        ].join("")
+      end
+
+      describe "#tarball_path" do
+        subject { downloader.tarball_path }
+        its(:extname) { is_expected.to eq(".zip") }
+        its("to_path.length") { is_expected.to be_between(0, 255) }
+      end
+    end
   end
 
   describe Hbc::CurlPostDownloadStrategy do