keep PYTHONPATH when requested

This turns out to be needed by the testsuite of jepler_udecimal, which
needs to add the `jepler_udecimal` directory both to PYTHONPATH and
MICROPYPATH.

Author: jepler

tests/run-tests.py

@@ -829,6 +829,10 @@ def run_one_test(test_file):
                 "PATH": os.environ["PATH"],
                 "LANG": "en_US.UTF-8",
             }
+            # CIRCUITPY-CHANGE: --keep-path applies to PYTHONPATH as well
+            if args.keep_path and os.getenv("PYTHONPATH"):
+                e["PYTHONPATH"] += ":" + os.getenv("PYTHONPATH")
+
             # run CPython to work out expected output
             try:
                 output_expected = subprocess.check_output(