Prior to this change, every integration test which wanted its line
endings to be handled transparently across platforms, i.e. test with
the same input that has its platform's line feed characters, converting
the line endings was up to each individual test by calling the
`platform_line` helper function. This significantly increases the amount
of boilerplate one has to copy between all the tests.
However, there are some test cases that need to exert strict control
over the exact input text without being manipulated behind the scenes by
the test framework.
So, with this change, the line feed conversions are factored into
the `TestCase` struct. By default, line endings of the input text
are converted to the platform's native line feed ending, but one can
explicitly specify in their test case when the input text should be left
alone and tested as is.
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