ChangeEmailValidator: add more sophisticated test cases

This does not do any functional change to the uploadvalidator, but adds
testcases of interest. Such are rejecting a certain username pattern
from a domain, while still allowing the domain.

Bug: 259663353
Change-Id: I29b8b00292a017d3acad557121d01d8f9a6ac54f
Signed-off-by: Matthias Maennich <maennich@google.com>