This allows the test to run on windows, and also makes writing compilation database tests easier in general. See D23532 for more information.
This sed call substitutes placeholders with absolute paths, which is an important part of this test. So we can't get rid of the sed call. Furthermore, the test verifies that clang-tidy doesn't crash when applying fixes (which it used to, IIRC), so I'm not sure this change doesn't make the test less informative.
If you need to test correct handling of GCC-style command lines in JSON compilation databases on Windows, we can add another test, but this one should stay as it is.
Please fix "No newline at end of file".