crypto: test that BoringCrypto is actually used

[FiloSottile]

#58803 is about BoringCrypto not being used for generating 4096-bit keys. In general, we don't have tests that check that BoringCrypto is actually used (when enabled) instead of Go crypto for operations that are in scope.

This is somewhat tricky to test. @rolandshoemaker suggested using the BoringCrypto counters.