Migrate to SwiftTesting

[0xTim]

Resolves #196 Also fixes an issue due to which the X5CVerifier would not verify a token with missing root in the x5c chain

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 90.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 76.82%. Comparing base (bba4fad) to head (851de24). Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
Sources/JWTKit/X5C/X5CVerifier.swift	88.46%	3 Missing :warning:

:exclamation: There is a different number of reports uploaded between BASE (bba4fad) and HEAD (851de24). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (bba4fad)	HEAD (851de24)
	4	3

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #197      +/-   ##
==========================================
- Coverage   81.91%   76.82%   -5.10%     
==========================================
  Files          55       55              
  Lines        1272     1290      +18     
==========================================
- Hits         1042      991      -51     
- Misses        230      299      +69     

Files with missing lines	Coverage Δ
Sources/JWTKit/JWTError.swift	93.84% <100.00%> (+0.40%)	:arrow_up:
Sources/JWTKit/X5C/X5CVerifier.swift	83.75% <88.46%> (+0.41%)	:arrow_up:

... and 5 files with indirect coverage changes

[ptoffy]

This also fixes an issue due to which the X5CVerifier would not verify a token with missing root in the x5c chain