TST: Use __tracebackhide__ to suppress unnecessary parts of tm assertions

[jbrockmendel]

Makes test output much more to the point.

Also gets rid of some backslashes, both for prettiness and to get rid of deprecation warnings in test logs.

[pep8speaks]

Hello @jbrockmendel! Thanks for submitting the PR.

• There are no PEP8 issues in the file pandas/util/testing.py !

[codecov]

Codecov Report

Merging #23307 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #23307      +/-   ##
==========================================
+ Coverage   92.22%   92.22%   +<.01%     
==========================================
  Files         169      169              
  Lines       50889    50897       +8     
==========================================
+ Hits        46932    46940       +8     
  Misses       3957     3957

Flag	Coverage Δ
#multiple	90.65% <100%> (ø)	⬆️
#single	42.29% <64.28%> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/util/testing.py	86.61% <100%> (+0.1%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ff55b32...547f2f7. Read the comment docs.

[jreback]

thanks!