Partial port of r78919 from trunk: add subdir argument to the findfile helper

Lib/test/support.py

@@ -387,12 +387,14 @@ if fp is not None:
     unlink(TESTFN)
 del fp
 
-def findfile(file, here=__file__):
+def findfile(file, here=__file__, subdir=None):
     """Try to find a file on sys.path and the working directory.  If it is not
     found the argument passed to the function is returned (this does not
     necessarily signal failure; could still be the legitimate path)."""
     if os.path.isabs(file):
         return file
+    if subdir is not None:
+        file = os.path.join(subdir, file)
     path = sys.path
     path = [os.path.dirname(here)] + path
     for dn in path: