Make testsuite windows compatible

[StarpTech]

First attempt to fix issues to run tests on windows

Outstanding issues:

• [ ] Absolute paths produced by vfile-reporter-pretty
• [ ] Some tests will fail with completely different errors on windows for instance:

should fail when writing to non-existent dirs
not ok 94 should report
  ---
    operator: deepEqual
    expected: |-
      [ null, 1, 'one.txt\n  1:1  error  Error: Cannot read output directory. Error:' ]
    actual: |-
      [ null, 1, 'one.txt\n  1:1  error  Error: Cannot write multiple files to single output: D:\\Repositories\\unified-engine\\test\\fixtures\\two-files\\three' ]
    at: onrun (D:\Repositories\unified-engine\test\output.js:471:10)
    stack: |-