Search backend: move file match limit test to result package

Created by: camdencheek

This moves TestFileMatch_Limit to the package that contains the type it is actually testing.

Side note: there is literally nothing in the textsearch package except tests and an empty file. It looks like the existing tests are targeting behavior that doesn't actually exist anymore (we don't have a textsearch job), but it's not immediately clear to me that the behavior being tested is covered elsewhere. Otherwise I'd just blow the package away.

Test plan

Just moving stuff around.