unittests: Do not import libnvim or headers in main process

Slows down unit tests much, but gets rid of as much preserved state as possible.

test/unit/profile_spec.lua

@@ -1,11 +1,13 @@
-local helpers = require 'test.unit.helpers'
+local helpers = require('test.unit.helpers')(after_each)
 local itp = helpers.gen_itp(it)
 
-local prof = helpers.cimport './src/nvim/profile.h'
+local cimport = helpers.cimport
 local ffi = helpers.ffi
 local eq = helpers.eq
 local neq = helpers.neq
 
+local prof = cimport('./src/nvim/profile.h')
+
 local function split(inputstr, sep)
   if sep == nil then
     sep = "%s"