tests/run-natmodtests.py: Automatically skip tests that are too large.

This follows a similar change made for `run-tests.py` in commit
2291045.  The change here uses the same
logic to detect if a natmod test is too big for the target (eg overflows
(I)RAM loading the native .mpy), by printing "START TEST" at the start of
the test.

Typical output is now something like this:

    ...
    pass  extmod/random_basic.py
    pass  extmod/random_extra_float.py
    pass  extmod/random_extra.py
    SKIP  extmod/random_seed_default.py
    LRGE  extmod/re1.py
    SKIP  extmod/re_debug.py
    pass  extmod/re_error.py
    pass  extmod/re_group.py
    pass  extmod/re_groups.py
    ...

and the tests that are too large are reported at the end, and written to
the `_result.json` file.

Signed-off-by: Damien George <damien@micropython.org>

Author: dpgeorge

tests/run-natmodtests.py

@@ -170,15 +170,26 @@ def run_tests(target_truth, target, args, resolved_arch):
             print("skip  {} - mpy file not compiled".format(test_file))
             continue
         test_script += bytes(injected_import_hook_code.format(test_module), "ascii")
+        test_script += b"print('START TEST')\n"
         test_script += test_file_data
 
         # Run test under MicroPython
         result_out, error = target.run_script(test_script)
 
         # Work out result of test
         extra = ""
+        result_out = result_out.removeprefix(b"START TEST\n")
         if error is None and result_out == b"SKIP\n":
             result = "SKIP"
+        elif (
+            error is not None
+            and error.args[0] == "exception"
+            and error.args[1] == b""
+            and b"MemoryError" in error.args[2]
+        ):
+            # Test had a MemoryError before anything (should be at least "START TEST")
+            # was printed, so the test is too big for the target.
+            result = "LRGE"
         elif error is not None:
             result = "FAIL"
             extra = " - " + str(error)
@@ -203,6 +214,8 @@ def run_tests(target_truth, target, args, resolved_arch):
             test_results.append((test_file, "pass", ""))
         elif result == "SKIP":
             test_results.append((test_file, "skip", ""))
+        elif result == "LRGE":
+            test_results.append((test_file, "skip", "too large"))
         else:
             test_results.append((test_file, "fail", ""))