tests: factor out zip and store helper functions

Created by: rvantonder

Searcher, replacer, and comby code use zip file / store functions that are duplicated. This PR factors out the functions and puts them in testutil/zip.go and testutil/store.go respectively. None of the functions are changed, they are only moved. Callers are updated.

Note: I don't want testutil to be a dumping ground for test functions; they only go here if they are shared by > 1 packages. Perhaps we can call this the testing package instead of testutil

Test plan: Semantics preserving. Affects only tests. Tests pass.