Use POST method for action=abusefilterevalexpression API requests

Needed as tests take arbitrary user input through a textarea,
which can get rather big. GET cuts it off sometimes.

Follow-up to I317f5d7c372710617a7a6526cd4613de9660a40a

Bug: 47298
Bug: 68767
Change-Id: Ibab02be4c4e5640a09a1a76a4aa3e1a9520f489d