Fix interop test generation

[mulmarta]

This PR fixes passive_client* interop test vectors:

• The key packages in the test vectors expired last week which broke tests for other implementations, so now we make the lifetime eternal.
• Test generation was broken due to multiple small bugs.

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

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 71.42857% with 8 lines in your changes are missing coverage. Please review.

Project coverage is 89.45%. Comparing base (8e74976) to head (12cac58).

Files	Patch %	Lines
mls-rs/src/client_builder.rs	55.55%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #113      +/-   ##
==========================================
- Coverage   89.47%   89.45%   -0.02%     
==========================================
  Files         173      173              
  Lines       31270    31296      +26     
==========================================
+ Hits        27978    27996      +18     
- Misses       3292     3300       +8     

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