Rename "warnings" to "pytest-warnings" in terminal output

Fix #970

testing/test_config.py

@@ -429,12 +429,12 @@ class TestWarning:
                 pass
         """)
         result = testdir.runpytest()
-        assert result.parseoutcomes()["warnings"] > 0
+        assert result.parseoutcomes()["pytest-warnings"] > 0
         assert "hello" not in result.stdout.str()
 
         result = testdir.runpytest("-rw")
         result.stdout.fnmatch_lines("""
-            ===*warning summary*===
+            ===*pytest-warning summary*===
             *WT1*test_warn_on_test_item*:5*hello*
         """)