test: only run Core tests upon pkg> test

[gdalle]

DI tests

• Remove all backend-specific dependencies and code from test env
• Upon pkg> test, only run the Core category of tests instead of every single backend and downstream test suite

[codecov]

Codecov Report

Attention: Patch coverage is 52.63158% with 9 lines in your changes missing coverage. Please review.

Project coverage is 97.58%. Comparing base (c20083f) to head (8282822).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
DifferentiationInterface/test/runtests.jl	30.76%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #659      +/-   ##
==========================================
- Coverage   97.58%   97.58%   -0.01%     
==========================================
  Files         111      112       +1     
  Lines        5552     5551       -1     
==========================================
- Hits         5418     5417       -1     
  Misses        134      134              

Flag	Coverage Δ
DI	98.74% <52.63%> (-0.01%)	⬇️
DIT	95.36% <ø> (ø)

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