BUG: runVimTestsSetup.vim isn't sourced on Unix when invoked through runVimTests.sh (with the .sh file extension).

I didn't catch this because I have a private runVimTests wrapper script,
and don't actually use the system-specific setup on Linux.

Fixes #6.
Tests: Add tests for sourcing of system-specific setup (runVimTestsSetup.vim).

Put the test into a subdir so that the expected results for the testdir
tests do not need to be adapted.