Run tests in Parallel where possible

[toqueteos]

There was no issue addressing this.

Changes proposed in this pull request:

• Make most of the tests run in parallel as they are totally independent from one another. Untouched tests (depend on os):
  • oslib_test.go (filesystem)
  • baselib_test.go (use os.Getenv)
  • script_test.go (use os.Chdir)

This change is very noticeable in CI specially when -race is enabled and/or running in GitHub Actions (tests can easily take 30+ seconds to run).

Here's some timing information (my local machine):

• Old: 7.674s (avg 12 runs)
• New: 2.669s (avg 12 runs)