Add partial support for running Parsoid selser tests
* This patch only adds support for
- selser manual edits tests
- selser changetree [5] tests
Both these types of selser tests are known statically.
* This doesn't yet add support for selser auto-edit tests since
they cannot be determined statically.
There is some code already in place in the test runner to apply
change trees, but it will all likely get refactored in a future
patch.
Bug: T270311
Depends-On: I3be7826e08be120043f3bcd9151bade930b65fb7
Change-Id: Ic824dae6ee49867c766d40c11acfda117c7e2689