Strategy: Expand test coverage

Between this commit and the previous one, this brings test coverage
for `Livecheck::Strategy` up to 98.18% line coverage and 97.22%
branch coverage. The only uncovered areas are some Sorbet `params`
calls (which I'm not sure how to cover) and a conditional `break` in
`page_headers` that will be refactored away in the future.

The increased coverage is primarily in areas that weren't covered
before because they call methods that make network requests. I worked
around this with stubs and doubles, so we can test this code to some
degree. I plan to expand this approach to other areas in livecheck
that aren't covered for the same reason and that should significantly
increase test coverage (along with some other test improvements that
I have lined up).

Library/Homebrew/test/livecheck/strategy_spec.rb

@@ -105,25 +105,50 @@ RSpec.describe Homebrew::Livecheck::Strategy do
     it "returns the Strategy module represented by the Symbol argument" do
       expect(strategy.from_symbol(:page_match)).to eq(Homebrew::Livecheck::Strategy::PageMatch)
     end
+
+    it "returns `nil` if the argument is `nil`" do
+      expect(strategy.from_symbol(nil)).to be_nil
+    end
   end
 
   describe "::from_url" do
-    let(:url) { "https://sourceforge.net/projects/test" }
+    let(:sourceforge_url) { "https://sourceforge.net/projects/test" }
 
-    context "when no regex is provided" do
+    context "when a regex or `strategy` block is not provided" do
       it "returns an array of usable strategies which doesn't include PageMatch" do
-        expect(strategy.from_url(url)).to eq([Homebrew::Livecheck::Strategy::Sourceforge])
+        expect(strategy.from_url(sourceforge_url)).to eq([Homebrew::Livecheck::Strategy::Sourceforge])
       end
     end
 
-    context "when a regex is provided" do
+    context "when a regex or `strategy` block is provided" do
       it "returns an array of usable strategies including PageMatch, sorted in descending order by priority" do
-        expect(strategy.from_url(url, regex_provided: true))
+        expect(strategy.from_url(sourceforge_url, regex_provided: true))
           .to eq(
             [Homebrew::Livecheck::Strategy::Sourceforge, Homebrew::Livecheck::Strategy::PageMatch],
           )
       end
     end
+
+    context "when a `strategy` block is required and one is provided" do
+      it "returns an array of usable strategies including the specified strategy" do
+        # The strategies array is naturally in alphabetic order when all
+        # applicable strategies have the same priority
+        expect(strategy.from_url(url, livecheck_strategy: :json, block_provided: true))
+          .to eq([Homebrew::Livecheck::Strategy::Json, Homebrew::Livecheck::Strategy::PageMatch])
+        expect(strategy.from_url(url, livecheck_strategy: :xml, block_provided: true))
+          .to eq([Homebrew::Livecheck::Strategy::PageMatch, Homebrew::Livecheck::Strategy::Xml])
+        expect(strategy.from_url(url, livecheck_strategy: :yaml, block_provided: true))
+          .to eq([Homebrew::Livecheck::Strategy::PageMatch, Homebrew::Livecheck::Strategy::Yaml])
+      end
+    end
+
+    context "when a `strategy` block is required and one is not provided" do
+      it "returns an array of usable strategies not including the specified strategy" do
+        expect(strategy.from_url(url, livecheck_strategy: :json, block_provided: false)).to eq([])
+        expect(strategy.from_url(url, livecheck_strategy: :xml, block_provided: false)).to eq([])
+        expect(strategy.from_url(url, livecheck_strategy: :yaml, block_provided: false)).to eq([])
+      end
+    end
   end
 
   describe "::post_args" do