test: refactor flag check

Refactor test-vm-run-in-new-context so that check for `--expose-gc` flag
will not run afoul of an upcoming lint rule that checks that string
literals are not used for the `message` argument of
`assert.strictEqual()`.

Backport-PR-URL: https://github.com/nodejs/node/pull/22888
PR-URL: https://github.com/nodejs/node/pull/22849


Reviewed-By: Teddy Katz <teddy.katz@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>