Add flatc python tests to CI (#7437)

tests/flatc/main.py

@@ -1,6 +1,11 @@
 #!/usr/bin/env python3
 
+import sys;
+
 from flatc_test import run_all
 from flatc_cpp_tests import CppTests
 
-run_all(CppTests)
+passing, failing = run_all(CppTests)
+
+if failing > 0:
+  sys.exit(1)