bpo-44131: Test Py_FrozenMain() (GH-26126)

* Add test_frozenmain to test_embed
* Add Programs/test_frozenmain.py
* Add Programs/freeze_test_frozenmain.py
* Add Programs/test_frozenmain.h
* Add make regen-test-frozenmain
* Add test_frozenmain command to Programs/_testembed
* _testembed.c: add error(msg) function

Programs/freeze_test_frozenmain.py

@@ -0,0 +1,48 @@
+import marshal
+import tokenize
+import os.path
+import sys
+
+PROGRAM_DIR = os.path.dirname(__file__)
+SRC_DIR = os.path.dirname(PROGRAM_DIR)
+
+
+def writecode(fp, mod, data):
+    print('unsigned char M_%s[] = {' % mod, file=fp)
+    indent = ' ' * 4
+    for i in range(0, len(data), 16):
+        print(indent, file=fp, end='')
+        for c in bytes(data[i:i+16]):
+            print('%d,' % c, file=fp, end='')
+        print('', file=fp)
+    print('};', file=fp)
+
+
+def dump(fp, filename, name):
+    # Strip the directory to get reproducible marshal dump
+    code_filename = os.path.basename(filename)
+
+    with tokenize.open(filename) as source_fp:
+        source = source_fp.read()
+        code = compile(source, code_filename, 'exec')
+
+    data = marshal.dumps(code)
+    writecode(fp, name, data)
+
+
+def main():
+    if len(sys.argv) < 2:
+        print(f"usage: {sys.argv[0]} filename")
+        sys.exit(1)
+    filename = sys.argv[1]
+
+    with open(filename, "w") as fp:
+        print("// Auto-generated by Programs/freeze_test_frozenmain.py", file=fp)
+        frozenmain = os.path.join(PROGRAM_DIR, 'test_frozenmain.py')
+        dump(fp, frozenmain, 'test_frozenmain')
+
+    print(f"{filename} written")
+
+
+if __name__ == "__main__":
+    main()