Add failing tests for `popen_write`
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).

reitermarkus/brew,reitermarkus/brew,Homebrew/brew,EricFromCanada/brew,mahori/brew,MikeMcQuaid/brew,sjackman/homebrew,sjackman/homebrew,JCount/brew,nandub/brew,nandub/brew,nandub/brew,JCount/brew,JCount/brew,konqui/brew,konqui/brew,vitorgalvao/brew,EricFromCanada/brew,JCount/brew,mahori/brew,reitermarkus/brew,Homebrew/brew,vitorgalvao/brew,MikeMcQuaid/brew,vitorgalvao/brew,claui/brew,Homebrew/brew,MikeMcQuaid/brew,reitermarkus/brew,Homebrew/brew,EricFromCanada/brew,EricFromCanada/brew,konqui/brew,mahori/brew,mahori/brew,claui/brew,vitorgalvao/brew,konqui/brew,sjackman/homebrew,claui/brew,nandub/brew,sjackman/homebrew,MikeMcQuaid/brew,claui/brew