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.
3. Added bug fix to whatsnew
4. Added a comment in the test

Didn't follow the contributing guidelines carefully in PR #11771 and hence this new PR. 

merged via https://github.com/pydata/pandas/commit/4fbf5e51f920dd1019360495e7eecd94016a7a9d

thanks!
