Add tests for shebang mixins

[dduugg]

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Adds test for a pair of mixins, which previously broke when being refactored due to a lack of test coverage for the instance case of module_function. (See prior comment)

These are my first tests that make use of files and formulae, so please apply an appropriate amount of scrutiny (I didn't find examples that directly imitated what I was attempting to test).

cc @Bo98 @MikeMcQuaid

[MikeMcQuaid]

Suggested change

	let(:file) { File.open("#{TEST_TMPDIR}/perl-shebang", "w") }
	let(:file) { Pathname.new("perl-shebang") }

as this was technically a Pathname not File. Similarly, I think TEST_TMPDIR may be implicit.

Generally it's considered 👎🏻 to use File.open without the block form.

[reitermarkus]

We have mktmpdir which creates a directory that is automatically cleaned after a test.

[github-actions]

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