TST: bare-bones fixture for timedelta array tests

[jbrockmendel]

Broken off from #23140. The fixture needs to be fleshed out, but for now I just want to get it in place, since already other PRs need it.

• closes #xxxx
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[pep8speaks]

Hello @jbrockmendel! Thanks for submitting the PR.

• There are no PEP8 issues in the file pandas/tests/arrays/test_datetimelike.py !

[codecov]

Codecov Report

Merging #23207 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #23207   +/-   ##
=======================================
  Coverage   92.19%   92.19%           
=======================================
  Files         169      169           
  Lines       50950    50950           
=======================================
  Hits        46975    46975           
  Misses       3975     3975

Flag	Coverage Δ
#multiple	90.62% <ø> (ø)	⬆️
#single	42.27% <ø> (ø)	⬆️

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[jreback]

thanks!