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

tests/run-tests.py

@@ -2794,9 +2794,10 b' class TestRunner(object):'
             if not (os.path.basename(t).startswith(b'test-')
                     and (t.endswith(b'.py') or t.endswith(b'.t'))):
 
-                m = testcasepattern.match(t)
+                m = testcasepattern.match(os.path.basename(t))
                 if m is not None:
-                    t, _, casestr = m.groups()
+                    t_basename, _, casestr = m.groups()
+                    t = os.path.join(os.path.dirname(t), t_basename)
                     if casestr:
                         case = casestr.split(b'#')
                 else: