Remove run_tests from public API

Users should not call run_tests() directly in their submission_tests
files. The task.py script handles calling it after loading the module.
By not exporting run_tests from the package, users will get an
ImportError if they try to use it incorrectly.

Author: Nils Bars

ref_utils/__init__.py

@@ -24,7 +24,6 @@
     "submission_test",
     "add_environment_test",
     "add_submission_test",
-    "run_tests",
     # Process
     "drop_privileges",
     "run",
@@ -55,7 +54,6 @@
     add_environment_test,
     add_submission_test,
     environment_test,
-    run_tests,
     submission_test,
 )
 from .process import (