Remove explicit path to which
Remove explicit path to which

It is possible that on *nix systems, `which` program isn't located in
`/usr/bin/which`. Thus, just use `which` and assume that it's in `PATH`.
At least on NixOS, `which` is located in a complex path in nix store,
thus the path to it cannot be hardcoded.

tk0miya/testing.common.database