Fix benchmark failure on FIPS builds

[bigbrett]

Description

Fixes bug where tls_bench can fail on FIPS builds in a multithreaded environment because of simultaneous attempts by client and server threads to run a CAST on-the-fly during the TLS handshake. This usually manifests itself as a KAT failure in ECC or AES, though I have seen a few other spurious error messages pop up depending on timing.

Bug reproduction

Try and run tls_bench on a multithreaded system on master and it should fail and report a KAT failure. Running on this branch should resolve the issue.

[dgarske]

Retest this please

[dgarske]

Retest this please

[dgarske]

Can't seem to get it to re-test or pass this macOS check in GitHub CI. I went ahead and rebased and force pushed. Let's see if we still get a failure.