Tests: Extend &runtimepath to also use vimtest.vim from the repository and (optionally) VimTAP from a repo next to it.

Without my private runVimTestsSetup.vim, the test suite has to work with the default ~/vimfiles location, and therefore I need to provide access to the testing scripts used by the self-tests.
Tests: Also prefer to use runVimTests.cmd from the repository.

For consistency with the Unix version, and also now required by tests/runVimTests/runVimTestsSetup/testRunVimTestsSetup.vim.