lit googletest.py: Don't raise StopIteration in generator

The intention here seems to be to end the generator function, but with
modern Python, raising StopIteration causes a runtime error
(https://www.python.org/dev/peps/pep-0479/).

Differential revision: https://reviews.llvm.org/D79169

llvm/utils/lit/lit/formats/googletest.py

@@ -41,7 +41,7 @@ class GoogleTest(TestFormat):
             litConfig.warning(
                 "unable to discover google-tests in %r: %s. Process output: %s"
                 % (path, sys.exc_info()[1], exc.output))
-            raise StopIteration
+            return
 
         nested_tests = []
         for ln in output.splitlines(False):  # Don't keep newlines.