Add failing tests for popen_write

When using `popen_write`, the expectation is to return the
standard output of the child process.

This expectation is evident in how `safe_popen_write` is written:

```
  def self.safe_popen_write(*args, **options, &block)
    output = popen_write(*args, **options, &block)
    return output if $CHILD_STATUS.success?

    raise ErrorDuringExecution.new(args, status: $CHILD_STATUS, output: [[:stdout, output]])
  end
```

However, no code has been written to actually *obtain* that output
from the child process. The side effects of that are described in
issue #8244. [1]

[1]: https://github.com/Homebrew/brew/issues/8244

The newly-added tests reveal that `popen_write` only returns the
number 4 instead of the expected standard output.

For example, given a file `foo` with the content `Foo\n`, one test
calls `popen_write` with `cat foo -` and an input of `Bar`.
The expected output would be `Foo\nBar\n` but the actual output is
the number 4 (which is what Ruby’s `IO#write` method returns).

Library/Homebrew/test/utils/popen_spec.rb

@@ -26,11 +26,49 @@ describe Utils do
   end
 
   describe "::popen_write" do
-    it "with supports writing to a command's standard input" do
+    let(:foo) { mktmpdir/"foo" }
+
+    before { foo.write "Foo\n" }
+
+    it "supports writing to a command's standard input" do
       subject.popen_write("grep", "-q", "success") do |pipe|
-        pipe.write("success\n")
+        pipe.write "success\n"
       end
       expect($CHILD_STATUS).to be_a_success
     end
+
+    it "returns the command's standard output before writing" do
+      child_stdout = subject.popen_write("cat", foo, "-") do |pipe|
+        pipe.write "Bar\n"
+      end
+      expect($CHILD_STATUS).to be_a_success
+      expect(child_stdout).to eq <<~EOS
+        Foo
+        Bar
+      EOS
+    end
+
+    it "returns the command's standard output after writing" do
+      child_stdout = subject.popen_write("cat", "-", foo) do |pipe|
+        pipe.write "Bar\n"
+      end
+      expect($CHILD_STATUS).to be_a_success
+      expect(child_stdout).to eq <<~EOS
+        Bar
+        Foo
+      EOS
+    end
+
+    it "supports interleaved writing between two reads" do
+      child_stdout = subject.popen_write("cat", foo, "-", foo) do |pipe|
+        pipe.write "Bar\n"
+      end
+      expect($CHILD_STATUS).to be_a_success
+      expect(child_stdout).to eq <<~EOS
+        Foo
+        Bar
+        Foo
+      EOS
+    end
   end
 end