Track down regression in nargo test speed in noir-bignum library

[TomAFrench]

There's been a significant regression in the time taken to run the test suite on https://github.com/noir-lang/noir-bignum.

On a recent commit we end up taking ~28x the time to run the test suite relative to the 1.0.0-beta.1 release.

We should determine where this regression came in and address it.

[TomAFrench]

This seems to be caused by #6969. Prior to nightly-2025-01-08 running nargo test on the current master of bignum run only 62 tests but after this nightly version we end up running 167 tests.

I'm closing this as the "better performance" we were getting before was due to us not running the full test suite.