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.
Don't do args checking in a test. If it's in the test code, use `Assert.NotNull`. If it's in the actual test, let it null ref.