Use LuaJIT FFI to create char pointer.
Validate output with utf_ptr2char(), vim_iswordc() and vim_iswordp().
Use const for LuaJIT string-to-char conversion.
Used
sed -r -i -e '/ helpers =/ s/$/\nlocal itp = helpers.gen_itp(it)/; s/^(\s*)it\(/\1itp(/' test/unit/**/*_spec.lua
to alter all tests. Locally they all run fine now.
Reasoning:
1. General: state from one test should not affect other tests.
2. Local: travis build is failing with something which may be an output of
garbage collector. This should prevent state of the garbage collector from
interferring as well.
