c2a: Only use postsubmit when configured

Previous logic would place tests in both presubmit and postsubmit if the
postsubmit_tests configuration was not specified. This is both
unnecessary and wastes resources because presubmit tests are also run in
postsubmit.

Bug: 236006683
Test: Regenerate all TEST_MAPPINGs, manually check postsubmit removal
Change-Id: Ie8ff0e351776711e07bf7646efd916d970254e9c

scripts/update_crate_tests.py

@@ -298,6 +298,12 @@ class TestMapping(object):
                         continue
                     if test not in config['postsubmit_tests'] and 'postsubmit' in test_group:
                         continue
+                else:
+                    if 'postsubmit' in test_group:
+                        # If postsubmit_tests is not configured, do not place
+                        # anything in postsubmit - presubmit groups are
+                        # automatically included in postsubmit in CI.
+                        continue
                 if test in TEST_OPTIONS:
                     test_mapping[test_group].append({"name": test, "options": TEST_OPTIONS[test]})
                 else: