test: spawn_wait() starts a non-RPC Nvim process

Problem:
Can't use `n.clear()` to test non-RPC `nvim` invocations. So tests end
up creating ad-hoc wrappers around `system()` or `jobstart()`.

Solution:
- Introduce `n.spawn_wait()`
- TODO (followup PR): Rename `n.spawn()` and `n.spawn_wait()`.
  It's misleading that `n.spawn()` returns a RPC session...

runtime/lua/coxpcall.lua

@@ -1,6 +1,10 @@
 -------------------------------------------------------------------------------
+-- (Not needed for LuaJIT or Lua 5.2+)
+--
 -- Coroutine safe xpcall and pcall versions
 --
+-- https://keplerproject.github.io/coxpcall/
+--
 -- Encapsulates the protected calls with a coroutine based loop, so errors can
 -- be dealed without the usual Lua 5.x pcall/xpcall issues with coroutines
 -- yielding inside the call to pcall or xpcall.