test: simplify assertion in http2 tests

In test-http2-timeout-large-write.js and
test-http2-timeout-large-write-file.js:

Use assert.ok() on a boolean that the test itself creates and sets,
rather than assert.strictEqual(). This allows us to use a static message
without running afoul of the upcoming "do not use string literals with
assert.strictEqual()" lint rule.

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>