Tests: add luacheck for linting tests

Source: https://github.com/mpeterv/luacheck
Docs:   http://luacheck.readthedocs.org/en/0.12.0/index.html

Run via "make testlint".

test/.luacheckrc

@@ -0,0 +1,4 @@
+-- vim: ft=lua
+
+std = '+luajit +busted'
+cache = true