Add TagHelper functional test.

- Added an end-to-end test that verifies all content behaviors and functionalities of tag helpers.
- Added some common user scenarios to verify that the system works how we expect.

#1116

**Note:** Wasn't sure to what extent I should hold back on testing these so they may be too extensive.
nit: If you want to add comments make sure they explain why you are doing something, not what you are doing. What the code is doing should be clear from reading the code itself.