test: disallow explict use of "default" policy in tests

[toidiu]

Description of changes:

As a modern TLS library, s2n-tls aims to provide sane default. To uphold this promise, we are planning to add TLS1.3 support to the "default" and "default_fips" policies. However, making this change can result in broken tests since connections will now negotiate TLS1.3 and we have to assume that old tests were written to assume that TLS1.2 would be negotiated.

To prevent this regression, this PR attempts to find "explicit" usage of the "default" policy (implicit detection will be a followup)("default_fips" will be a followup) and replace it with the immutable numbered policy 20240501. I audited both the C and Rust codebase for "default" security policy usage, allowing for exceptions for tests which didnt care about the protocol.

Call-outs:

The detection logic can be viewed by looking at the detection logic cleanup commit.

• Detection logic was done in function s2n_find_security_policy_from_version in file s2n_security_policies.c
  • matches_default: bail if code attempted to use the "default" security policy
  • dbg_bail: allow exception for test that really do want to test the "default" policy
  • dbg_config_init: allow exceptions when creating a new s2n_config_new()
  • s2n_is_initialized: allow exception for s2n_init() to create the default static config
    The PR also adds an additional check to grep_simple_mistakes.sh
• simple grep: prevent future explicit use of "default" in C code and DEFAULT in Rust code

Testing:

Tests should continue to pass.

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