Add fuzz testing for PKCS7_verify

[justsmth]

Description of changes:

• Add fuzz testing around PKCS7_verify

Call-outs:

• The only human-readable changes are in the first commit: bb68163

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

Attention: Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 78.76%. Comparing base (51ae4b1) to head (f5e947c).

Files with missing lines	Patch %	Lines
crypto/pkcs7/pkcs7.c	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2051      +/-   ##
==========================================
- Coverage   78.77%   78.76%   -0.02%     
==========================================
  Files         598      598              
  Lines      103691   103695       +4     
  Branches    14734    14734              
==========================================
- Hits        81682    81672      -10     
- Misses      21357    21373      +16     
+ Partials      652      650       -2     

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