Make AWS_LC_fips_failure_callback optional in builds with AWSLC_FIPS_FAILURE_CALLBACK

[andrewhop]

Description of changes:

This makes the FIPS callback optional when AWS-LC is built with AWSLC_FIPS_FAILURE_CALLBACK which turns it on. Previously if AWS-LC was built with AWSLC_FIPS_FAILURE_CALLBACK then any application that might link with AWS-LC would need to define the callback or AWS-LC would abort. Turns out that's hard to ensure, this change goes back to the old behavior:

• Regular FIPS build of AWS-LC doesn't require a callback and always aborts
• FIPS build with AWSLC_FIPS_FAILURE_CALLBACK allows a callback to be defined and AWS-LC will call that and not abort
• FIPS build with AWSLC_FIPS_FAILURE_CALLBACK and no callback AWS-LC aborts

Call-outs:

I had to remove EXPECT_TRUE(EVP_PKEY_keygen(ed_ctx.get(), &ed_key)); because that keygen call should only happen after the check for the environment variable and now we're testing both paths.

Testing:

I created a new test target that partially replicates our internal FIPS callback build.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.