fish: fix and enable `fishConfig` test

`nix-build -A fish.tests.fishConfig` will now test that the
`fish_config` tool would start up properly. Previously, this test was
effectively disabled due to `withTests` being stubbed out.

I don't think this test ever truly worked, because the generated
temporary file would be cleaned up automatically (or maybe this
"automatically" changed between versions). The solution to this is to
add `delete=False` to the `NamedTemporaryFile` function call, to keep
the temporary file around in order to grep its contents for the expected
output.