Support `// Env` in the python test runner

[joyeecheung]

Something I found very useful in V8's test runner (our Python test runner is a fork of a very old version of this) is the support of the // Environment variables comment (similar to the // Flags comment, this instructs the test runner to pass a particular flag when running the test)

node/deps/v8/test/intl/default_locale.js

Line 5 in 7b48713

// Environment Variables: LC_ALL=de

The way it's implemented in V8 can be found in e.g. here

node/deps/v8/test/intl/testcfg.py

Line 34 in 17a74dd

ENV_PATTERN = re.compile(r"//\s+Environment Variables:(.*)")

We can probably just port similar logic to our fork in e.g. here

node/test/testpy/__init__.py

Line 61 in a790901

flags_match = FLAGS_PATTERN.search(source)

It would be handy to leave more logs in flaky tests via the use of // Env: NODE_DEBUG=foo or // Env: NODE_DEBUG_NATIVE=foo - trying to set the flag in the entire test job or the entire test suite would be uncanny since many tests also check that stdout/stderr is empty, and we probably only want to do this for selected flaky tests that don't care about stdout/stderr so that they leave more traces in the CI logs. This would also help simplifying many tests that only spawn child processes to customize the environment varaibles.