QA: Fix edit_existing.feature test

Make edit title test work.
Make edit post test work.

Ensure these tests create a new post for the purpose of the test to
avoid edit conflicts and ensure we do not need to resort to horrible
hacks to test.

Change-Id: I25580dce906cb04cc8e4d4d431d56535ac0c3321
Merge "QA: Fix edit_existing.feature test" into frontend-rewrite