test: improve performance of test-crypto-timing-safe-equal-benchmarks

Using `eval()` rather than `require()`'ing a fixture and deleting it
from the cache results in a roughtly 10x improvement in running time.

Fixes: https://github.com/nodejs/node/issues/25984
Refs: https://github.com/nodejs/node/issues/26229

PR-URL: https://github.com/nodejs/node/pull/26237


Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Yang Guo <yangguo@chromium.org>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Teddy Katz <teddy.katz@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>