tests: add -k to test scripts matching keywords
argument is a space-separated list of keywords that are searched for
in the name and body of each test. This makes it easy to run only
tests related to tags, hgweb, revert, etc. (eg -k "tag hgweb revert").
# should print nothing
# should fail
abort: path 'b/x' is inside repo 'b'
abort: path 'b/x' is inside repo 'b'
# should fail
abort: path 'b/x' is inside repo 'b'
# should arguably print nothing
# should fail
abort: path 'b/a' is inside repo 'b'