run-test: replace backslashes in TESTDIR

This may cause troubles in MSYS on Windows.

tests/run-tests.py

@@ -1197,7 +1197,7 b' def main():'
             del os.environ[k]
 
     global TESTDIR, HGTMP, INST, BINDIR, PYTHONDIR, COVERAGE_FILE
-    TESTDIR = os.environ["TESTDIR"] = os.getcwd()
+    TESTDIR = os.environ["TESTDIR"] = os.getcwd().replace('\\', '/')
     if options.tmpdir:
         options.keep_tmpdir = True
         tmpdir = options.tmpdir