hghave: fix 'rmcwd' to ensure temporary directory is removed stable

hghave: fix 'rmcwd' to ensure temporary directory is removed

On platforms where cwd can't be removed, it should try rmdir() after chdir
to the original cwd.

--- a/tests/hghave.py	Mon Oct 31 13:43:48 2016 +0100
+++ b/tests/hghave.py	Tue Nov 01 21:14:33 2016 +0900
@@ -360,6 +360,11 @@
         return False
     finally:
         os.chdir(ocwd)
+        # clean up temp dir on platforms where cwd can't be removed
+        try:
+            os.rmdir(temp)
+        except OSError:
+            pass
 
 @check("tla", "GNU Arch tla client")
 def has_tla():