PEP8 line length fixes in testing_commands.py

...so that PyLint stops yelling at me
Adds a --component (or -c) argument to test-unit

Example usage:

`./mach test-unit -c style`
Last second bug catch
Corrected case of Document#characterSet

Resolves #4251
Use .clone() instead of.as_slice().to_string()
Update content tests for Document#characterSet
Split up the InputButton variant
Implement form control mutability, rename FormOwner -> FormControl
Add stub Activatable trait
Add trusted setter to Event
Implement basic (unhooked) framework for element activation
Hook up authentic click activation to the script task
Some reorganization of activation code:

 - Make method name apply to trait implementor better (When a user agent is to run post-click activation steps on an element, it must run the activation behavior defined for that element)
 - Mention invariants and conditions on authentic_click_activation
Move InputSubmit to Activatable
Move InputCheckbox to Activatable
Refactor code for fetching radio siblings
Move InputRadio to Activatable
Hook up synthetic click activation to script_task and <>.click()
Implement implicit form submission
Address review comments