[PATCH] [libcxx] Add better support for custom test runners.

[Eric Fiselier]

================
Comment at: test/lit.cfg:204
@@ -184,3 +203,3 @@
                 try:
-                    os.remove(exec_path)
+                    self._clean(exec_path)
                 except:
----------------
`exec_path` is always created so it should always be removed.  If creating the file doesn't belong to a customization point then neither should cleaning it up. 

Small nit: Will there ever be a case we want `_clean(...)` to throw? Should exception handling be the responsibility of `_clean(...)`?

================
Comment at: test/lit.cfg:529
@@ -508,1 +528,3 @@
+    print 'Using configuration variant: %s' % cfg_variant
+configuration = globals()['%sConfiguration' % cfg_variant](lit_config, config)
 configuration.configure()
----------------
Is there a prettier way to right this line? Just so I understand `globals()['%sConfiguration' % cfg_variant]` looks up and evaluates to a type with the name `%sConfiguration`?

http://reviews.llvm.org/D6373