test: Extend {unit,functional}.helpers with global helpers

Automatically include all "global helper" util functions in the
unit.helpers and functional.helpers and modules.  So tests don't need to
expicitly do:

    local global_helpers = require('test.helpers')

test/functional/api/server_requests_spec.lua

@@ -1,7 +1,6 @@
 -- Test server -> client RPC scenarios. Note: unlike `rpcnotify`, to evaluate
 -- `rpcrequest` calls we need the client event loop to be running.
 local helpers = require('test.functional.helpers')(after_each)
-local Paths = require('test.config.paths')
 
 local clear, nvim, eval = helpers.clear, helpers.nvim, helpers.eval
 local eq, neq, run, stop = helpers.eq, helpers.neq, helpers.run, helpers.stop
@@ -243,8 +242,8 @@ describe('server -> client', function()
         \ 'rpc': v:true
         \ }
       ]])
-      local lua_prog = Paths.test_lua_prg
-      meths.set_var("args", {lua_prog, 'test/functional/api/rpc_fixture.lua'})
+      meths.set_var("args", {helpers.test_lua_prg,
+                             'test/functional/api/rpc_fixture.lua'})
       jobid = eval("jobstart(g:args, g:job_opts)")
       neq(0, 'jobid')
     end)