Add compare-output-lines-by-subset flag to compiletest
For [ferrocene](github.com/ferrocene/) we have some compiletests that check the output of the cli arguments to the compiler, including printing things like the target list (`--print target-list`). Unfortunately those tend to change quite often so when we sync we end up with some outputs we have to re-bless constantly, even though the exact output doesn't really matter.
We added a new compiletest flag to aid writing these kinds of tests: `compare-output-lines-by-subset`. It checks whether the lines of the expected output are a subset (or equal) to the lines of the actual output. If the expected output is empty it will fail unless the actual output is also empty. We opened this PR hoping the flag might be helpful for other tests in the future (especially if CLI-related tests are added in the future in the rust-lang/rust repo itself).
a57fa08f48