Add silly debug flags for npm debugging

[rollecode]

This adds verbose debug flags in case of npm commands hang or behave unexpectedly.

[pfefferle]

Can you make it configurable by supporting the env var WPT_DEBUG?

https://github.com/WordPress/phpunit-test-runner/blob/master/.env.default#L62

You see an example here:

phpunit-test-runner/prepare.php

Line 33 in 6fc0d25

* Determines if the debug mode is enabled based on the 'WPT_DEBUG' environment variable.

[mindctrl]

I wonder if we still need this since #222 was solved with #225? If we still want it, I like the idea of making it configurable via WPT_DEBUG like @pfefferle suggested.

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[kittenkamala]

Okay, this needs further discussion.

1. Is this still needed?
2. Move forward with WPT_DEBUG?
3. Needs someone to update the PR to use WPT_Debug instead of the -ddd flag

[ekamran]

Picking up the questions above:

1. Still needed? The original hang in npm install gets stuck / hangs in reify:poly-fill-library #222 was fixed by a Node version bump in updated Node version to latest supported one #225, but that fixed only that incident. If npm hangs again there is still no output to diagnose it, so an opt-in verbose mode is still useful.

2. WPT_DEBUG: agree. The runner already has WPT_DEBUG wired in, so the npm commands now add the verbose flag only when it is set. Default behavior stays the same.

3. I opened Run npm verbosely when debug mode is enabled #340 with that approach, keeping credit to @rollecode for the original idea here.