Lower the cos test tolerance and fix the asin test seed

[ibmibmibm]

This PR follows the review of #1479.

test_cos() in test/test_sin_cos.cpp now uses a tolerance of 16 float eps, the same value that #1479 uses for test_sin(). The old value was 35 eps. Over 200000 random points in [-2π, 2π], the largest cos error for all decimal types is 5 eps.

test_asin_edge() in test/test_asin.cpp now seeds its generator with a fixed value. The old code seeded it from the clock, so a failure was not reproducible. This change also removes the unused time_point() helper, std::random_device, and <chrono>.

This PR does not change test/test_tan.cpp because #1479 already changes the seed there.

Both tests pass with GCC and Clang.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.7%. Comparing base (c4e5174) to head (ca990cd).

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #1484     +/-   ##
=========================================
- Coverage     98.7%   98.7%   -0.0%     
=========================================
  Files          315     315             
  Lines        26449   26441      -8     
  Branches      2253    2253             
=========================================
- Hits         26104   26096      -8     
  Misses         345     345             

Files with missing lines	Coverage Δ
test/test_asin.cpp	100.0% <100.0%> (ø)
test/test_sin_cos.cpp	100.0% <100.0%> (ø)

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