run-tests: fix TESTDIR if testdescs are absolute paths

Commit a18eef03d879 made TESTDIR be the location of the arguments that were
passed to run-tests.py instead of just PWD.  It assumed that these tests were
specified using relative paths, so if pwd was /tmp/foo, and the first argument
was /tmp/baz, it would set TESTDIR to /tmp/foo//tmp/baz.

Differential Revision: https://phab.mercurial-scm.org/D1433

tests/run-tests.py

@@ -2356,9 +2356,9 b' class TestRunner(object):'
         # assume all tests in same folder for now
         if testdescs:
             pathname = os.path.dirname(testdescs[0]['path'])
-            if pathname and not osenvironb[b'TESTDIR'].endswith(b'/'):
-                osenvironb[b'TESTDIR'] += b'/'
-            osenvironb[b'TESTDIR'] += pathname
+            if pathname:
+                osenvironb[b'TESTDIR'] = os.path.join(osenvironb[b'TESTDIR'],
+                                                      pathname)
         if self.options.outputdir:
             self._outputdir = canonpath(_bytespath(self.options.outputdir))
         else: