Fix tests that assume X25519 will be negotiated

[alexw91]

Issues:

Description of changes:

Fixes tests that assume X25519 will be negotiated and will break when the default TLS supported group is changed to X25519MLKEM768.

Related to https://github.com/aws/aws-lc/pull/2531/files

Call-outs:

None

Testing:

Unit tests

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

❌ Patch coverage is 70.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.83%. Comparing base (5f1d0d2) to head (b832653).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
ssl/ssl_version_test.cc	62.50%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2682      +/-   ##
==========================================
- Coverage   78.83%   78.83%   -0.01%     
==========================================
  Files         667      667              
  Lines      114077   114089      +12     
  Branches    16046    16046              
==========================================
+ Hits        89935    89937       +2     
- Misses      23365    23378      +13     
+ Partials      777      774       -3     

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