tests/run-natmodtests.py: Don't allow imports from the cwd.

Make tests run in an isolated environment (i.e. `import io` would
otherwise get the `tests/io` directory).

This work was funded through GitHub Sponsors.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

tests/run-natmodtests.py

@@ -46,14 +46,14 @@ class __FS:
   def chdir(self, path):
     pass
   def stat(self, path):
-    if path == '__injected.mpy':
+    if path == '/__injected.mpy':
       return tuple(0 for _ in range(10))
     else:
       raise OSError(-2) # ENOENT
   def open(self, path, mode):
     return __File()
 os.mount(__FS(), '/__remote')
-os.chdir('/__remote')
+sys.path.insert(0, '/__remote')
 sys.modules['{}'] = __import__('__injected')
 """
 
@@ -111,9 +111,10 @@ def run_tests(target_truth, target, args, stats):
             test_file_data = f.read()
 
         # Create full test with embedded .mpy
+        test_script = b"import sys\nsys.path.remove('')\n\n"
         try:
             with open(NATMOD_EXAMPLE_DIR + test_mpy, "rb") as f:
-                test_script = b"__buf=" + bytes(repr(f.read()), "ascii") + b"\n"
+                test_script += b"__buf=" + bytes(repr(f.read()), "ascii") + b"\n"
         except OSError:
             print("----  {} - mpy file not compiled".format(test_file))
             continue