implement test_scalar_compat

[jbrockmendel]

More test organizing, this one focusing on tests.indexes.datetimes.

• continue collecting staggler arithmetic tests
• new test module for testing that DatetimeIndex.foo behaves like Timestamp.foo
• de-duplicate some tests scattered across modules
• parametrize a handful of tests

[codecov]

Codecov Report

Merging #19479 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #19479   +/-   ##
=======================================
  Coverage   91.62%   91.62%           
=======================================
  Files         150      150           
  Lines       48728    48728           
=======================================
  Hits        44645    44645           
  Misses       4083     4083

Flag	Coverage Δ
#multiple	89.99% <ø> (ø)	⬆️
#single	41.74% <ø> (ø)	⬆️

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[jreback]

thanks!