Move ADTypeCheckContext tests to a separate module

[mhauru]

They were being run when test utils was imported, which was not ideal, and hard to leave out. Now they are in a module of their own.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.39%. Comparing base (d2ea94a) to head (61e19f9).
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2383      +/-   ##
==========================================
- Coverage   86.45%   86.39%   -0.07%     
==========================================
  Files          22       22              
  Lines        1573     1573              
==========================================
- Hits         1360     1359       -1     
- Misses        213      214       +1     

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[penelopeysm]

Lovely, feel free to merge when CI passes :)

I wonder if we should have a merge queue for Turing as well?

[coveralls]

Pull Request Test Coverage Report for Build 11576558851

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.06%) to 78.685%

---

Files with Coverage Reduction	New Missed Lines	%
ext/TuringOptimExt.jl	1	98.04%

Totals
Change from base Build 11573267768:	-0.06%
Covered Lines:	1233
Relevant Lines:	1567

---

💛 - Coveralls