Do not auto-detect tests from vendor, recursively

[borekb]

After the recent change in #18 which added recursive scanning for tests (I think), I get a lot of tests that are not "mine" but from vendor directories:

I think the solution could be to automatically skip gitignored directories.

(This behavior is without any custom config. It's possible that I will be able to work around this by setting PHPUnit configuration via phpunit.args but I still think that by default, gitignored directories should not be scanned.)

[recca0120]

skip all vendor folder now