tests: support passing testcase after .t paths that have path separators

Authored by spectral.

Description

tests: support passing testcase after .t paths that have path separators

This probably could have been implemented by changing the regex above this bit
of code, but I wasn't sure if it would end up handling various OSes correctly,
so I decided to go with this version instead.

Previously:

$ tests/run-tests.py tests/test-ssh.t -l
running 2 tests using 2 parallel processes
..
# Ran 2 tests, 0 skipped, 0 failed.

$ tests/run-tests.py tests/test-ssh.t#sshv1 -l
running 0 tests using 0 parallel processes

# Ran 0 tests, 0 skipped, 0 failed.

Now:

$ tests/run-tests.py tests/test-ssh.t -l
running 2 tests using 2 parallel processes
..
# Ran 2 tests, 0 skipped, 0 failed.

$ tests/run-tests.py tests/test-ssh.t#sshv1 -l
running 1 tests using 1 parallel processes
.
# Ran 1 tests, 0 skipped, 0 failed.

Differential Revision: https://phab.mercurial-scm.org/D5535