RSA 8192 (slow) tests only for release build

[justsmth]

Description of changes:

• Avoid running slow RSA8192 tests on debug builds.

Call-outs:

This is similar to what we do for a couple of slow digest tests:

• aws-lc-rs/aws-lc-rs/tests/digest_test.rs

  Line 245 in 2298ca8

  #[cfg(not(debug_assertions))]

• aws-lc-rs/aws-lc-rs/tests/digest_test.rs

  Line 310 in 2298ca8

  #[cfg(not(debug_assertions))]

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.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.66%. Comparing base (c358484) to head (aab72ba).
Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #407      +/-   ##
==========================================
- Coverage   95.80%   92.66%   -3.14%     
==========================================
  Files          61       61              
  Lines        8143     8176      +33     
  Branches        0     8176    +8176     
==========================================
- Hits         7801     7576     -225     
- Misses        342      358      +16     
- Partials        0      242     +242     

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