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:

• https://github.com/aws/aws-lc-rs/blob/2298ca861234d4f43aecef2c7d7e822c60bc488a/aws-lc-rs/tests/digest_test.rs#L245
• https://github.com/aws/aws-lc-rs/blob/2298ca861234d4f43aecef2c7d7e822c60bc488a/aws-lc-rs/tests/digest_test.rs#L310

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 :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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