testsuite: fix failing pytest tests

gcc/testsuite/ChangeLog:

	* g++.dg/gcov/gcov.py: Fix failing pytests as gcov.json.gz
	  filename was changed in b777f228b481ae881a7fbb09de367a053740932c.
diff --git a/gcc/testsuite/g++.dg/gcov/gcov.py b/gcc/testsuite/g++.dg/gcov/gcov.py
index a8c4ea9..5137f3a 100644
--- a/gcc/testsuite/g++.dg/gcov/gcov.py
+++ b/gcc/testsuite/g++.dg/gcov/gcov.py
@@ -5,6 +5,9 @@
 
 def gcov_from_env():
     # return parsed JSON content a GCOV_PATH file
-    json_filename = os.environ['GCOV_PATH'] + '.gcov.json.gz'
+    json_filename = os.environ['GCOV_PATH']
+    # strip extension
+    json_filename = json_filename[:json_filename.rindex('.')]
+    json_filename += '.gcov.json.gz'
     json_data = gzip.open(json_filename).read()
     return json.loads(json_data)