Update Makefile: Set LUA_PATH

[smatting]

Without the changes in this PR make test fails on my machine like this

$ make test
TEST_FILES=$TEST_FILES NEOGIT_LOG_LEVEL=error NEOGIT_LOG_CONSOLE="sync" GIT_CONFIG_GLOBAL=/dev/null GIT_CONFIG_SYSTEM=/dev/null NVIM_APPNAME=neogit-test nvim --headless -S "./tests/init.lua"
Error detected while processing command line..script /home/stefan/repos/neogit/tests/init.lua:
E5113: Error while calling lua chunk: /home/stefan/repos/neogit/tests/init.lua:1: module 'tests.util.util' not found:
...

[CKolkey]

Cool :) Won't pretend to understand this incantation, but it works fine on my machine too.