refactor(general): make key derivers available in tests

[julio-lopez]

• Re-introduces the insecure, lightweight key deriver for testing only.
• Makes scrypt and pbkdf2 derivers available in tests as well.

[codecov]

Codecov Report

Attention: Patch coverage is 77.77778% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 77.05%. Comparing base (cb455c6) to head (dc61b78).
Report is 120 commits behind head on master.

Files	Patch %	Lines
internal/crypto/pb_key_deriver_insecure_testing.go	71.42%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3826      +/-   ##
==========================================
+ Coverage   75.86%   77.05%   +1.18%     
==========================================
  Files         470      479       +9     
  Lines       37301    28714    -8587     
==========================================
- Hits        28299    22125    -6174     
+ Misses       7071     4693    -2378     
+ Partials     1931     1896      -35     

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