test: add ability to skip common flag checks

Currently, `common/index.js` checks that our test files are
spawned with the flags specified in `// Flags:`, and re-spawns
with them if they are not found.

This can be *very* annoying, for example when debugging using
debuggers that attach to the parent process, or when intentionally
testing with flags that are different from the specified ones.

This adds a `NODE_SKIP_FLAG_CHECK` environment variable check.
Setting it to a non-empty value will skip the flag checks altogether.

PR-URL: https://github.com/nodejs/node/pull/27254


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Yuta Hiroto <hello@hiroppy.me>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>