Increase absolute tolerance to make CI pass with julia v1.9.3

[JoshuaLampert]

With julia v1.9.3 one test fails due to different l2 and linf errors, which is quite annoying. This PR simply increases the absolute tolerance as suggested by @ranocha in #1617 (comment). We need an absolute tolerance of 3e-10, which IMHO is quite high... I've also noticed a mismatch in the docstring of @test_trixi_include.
Fixes #1617.

[github-actions]

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[ranocha]

Thanks a lot! This change is fine with me.

If we have some time, it would be interesting to see whether the same kind of difference between Julia v1.9.0 and v1.9.3 persists if we choose controller = PIDController(0.55, -0.27, 0.05) as keyword argument in solve (this is a good controller for SSPRK43, see #1087).

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: +39.03% 🎉

Comparison is base (f098ea2) 49.33% compared to head (6a4f2c3) 88.36%.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1625       +/-   ##
===========================================
+ Coverage   49.33%   88.36%   +39.03%     
===========================================
  Files         414      414               
  Lines       33839    33888       +49     
===========================================
+ Hits        16692    29943    +13251     
+ Misses      17147     3945    -13202     

Flag	Coverage Δ
unittests	88.36% <ø> (+39.03%)	⬆️

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see 248 files with indirect coverage changes

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