[tests] disabled some tests when no Python 3.7+

tests/compliance/test_config

 PLUGIN:
+ENABLED_IF: %{read:../../../python-3.7-available}

tests/fc_script/test_config

@@ -3,4 +3,5 @@ COMMENT: Explicits the dependencies of {bin:frama-c-script} execution:
  PLUGIN: variadic,instantiate
 COMMENT: The library frama-c.analysis_scripts is part of the package frama-c
 COMMENT: and this dependency is implicit as the one to the needed python files.
+ENABLED_IF: %{read:../../../python-3.7-available}
 */

tests/jcdb/test_config

 PLUGIN:
+ENABLED_IF: %{read:../../../python-3.7-available}