CLN: test_timeseries_legacy, specialize assert_

GH6175: replacing "assert_" with specialized form assertEqual. WIP: have not adjusted any instances of assert_( (... == ...).all() )
CLN: test_timeseries, specialize assert_

GH6175: replacing "assert_" with specialized form assertEqual. WIP: have not adjusted any instances of assert_( (... == ...).all() )
Merge pull request #6201 from bwignall/assert_6175_3

Change more assert_'s to specialized forms