Update test framework: fix run_tests.py to support all test files, add auto-import-check for test files

node_modules/import-in-the-middle/test/README.md

@@ -0,0 +1,17 @@
+These tests are organized as follows:
+
+* Located in the `hook` directory if they use the `Hook` class.
+* Located in the `low-level` directory if they use the "low-level" API,
+  `addHook` and `removeHook`.
+* Other tests are in other adjacent directories.
+
+The tests can be run individually as Node.js programs with non-zero exit codes
+upon failures. They should be run with the following Node.js command-line
+options (assuming they're run from the project root):
+
+```
+--require ./test/version-check.js
+--experimental-loader ./test/generic-loader.mjs
+```
+
+The entire test suite can be run with `npm test`.