Fix the test suite so that it passes for Windows

[spydon]

Currently most of the tests fails when running the test suite on windows in the pipeline (well, on real machines too I expect).

The problem is that paths on windows use \ instead of /, which creates test fails such as these:

[spec_cli]: Expected: contains 'PASS  test/my_test.dart'
[spec_cli]:   Actual: ' PASS  test\\another_test.dart\n'
[spec_cli]:             ' PASS  test\\my_test.dart\n'
[spec_cli]:             '\n'
[spec_cli]:             'Test Suites: 2 passed, 2 total\n'
[spec_cli]:             'Tests:       3 passed, 3 total\n'
[spec_cli]:             'Time:        00:00:00\n'
[spec_cli]:             ''

I can't think of any other solution than to pass all the strings through some function that fixes the paths depending on platform in all the tests.