Write more tests for Crypto crate

[Hinton]

Type of change

- [ ] Bug fix
- [ ] New feature development
- [x] Tech debt (refactoring, code cleanup, dependency upgrades, etc)
- [ ] Build/deploy pipeline (DevOps)
- [ ] Other

Objective

Extend unit testing for the crypto crate.

Before you submit

• Please add unit tests where it makes sense to do so

[bitwarden-bot]

Checkmarx One – Scan Summary & Details – e4d86989-bf90-41c1-8aae-b5d83a3666d9

No New Or Fixed Issues Found

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (872a05b) 52.75% compared to head (953da67) 54.02%.

Files	Patch %	Lines
...rates/bitwarden-crypto/src/enc_string/symmetric.rs	97.10%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #570      +/-   ##
==========================================
+ Coverage   52.75%   54.02%   +1.27%     
==========================================
  Files         163      163              
  Lines        8381     8520     +139     
==========================================
+ Hits         4421     4603     +182     
+ Misses       3960     3917      -43     

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