BUG: [GH 11738] Fix for end_time when multiple of a frequency is requested
closes #11738 
1. Added a new test with the multiple of a frequency. 
2. Rearranged some tests to improve readability of the code.

looks good. can you add a whatsnew note (bug fix), ping when green
 @jreback Sorry, didn't follow contributing guidelines carefully in this PR. Please see PR #11779.
 ok, np. yeah, just keeping the original PR is much better / easier to follow the flow
