tests/run-multitests: Make paths more deterministic.

Allows running from a different directory, etc.

This work was funded by Planet Innovation.

tests/run-multitests.py

@@ -15,15 +15,24 @@
 import subprocess
 import tempfile
 
-sys.path.append("../tools")
+test_dir = os.path.abspath(os.path.dirname(__file__))
+
+if os.path.abspath(sys.path[0]) == test_dir:
+    # remove the micropython/tests dir from path to avoid
+    # accidentally importing tests like micropython/const.py
+    sys.path.pop(0)
+
+sys.path.insert(0, test_dir + "/../tools")
 import pyboard
 
 if os.name == "nt":
     CPYTHON3 = os.getenv("MICROPY_CPYTHON3", "python3.exe")
-    MICROPYTHON = os.getenv("MICROPY_MICROPYTHON", "../ports/windows/micropython.exe")
+    MICROPYTHON = os.getenv("MICROPY_MICROPYTHON", test_dir + "/../ports/windows/micropython.exe")
 else:
     CPYTHON3 = os.getenv("MICROPY_CPYTHON3", "python3")
-    MICROPYTHON = os.getenv("MICROPY_MICROPYTHON", "../ports/unix/build-standard/micropython")
+    MICROPYTHON = os.getenv(
+        "MICROPY_MICROPYTHON", test_dir + "/../ports/unix/build-standard/micropython"
+    )
 
 # For diff'ing test output
 DIFF = os.getenv("MICROPY_DIFF", "diff -u")
@@ -508,7 +517,7 @@ def main():
     cmd_args = cmd_parser.parse_args()
 
     # clear search path to make sure tests use only builtin modules and those in extmod
-    os.environ["MICROPYPATH"] = os.pathsep + "../extmod"
+    os.environ["MICROPYPATH"] = os.pathsep.join(("", ".frozen", "../extmod"))
 
     test_files = prepare_test_file_list(cmd_args.files)
     max_instances = max(t[1] for t in test_files)