test: assert coroutine.resume() result

`coroutine.resume()` works similarly to `pcall()` and catches any
errors. Ensure errors thrown in coroutine are propagated by asserting
on the values returned by `coroutine.resume()`.

lua/github-theme/_test/util.lua

@@ -10,7 +10,7 @@ function M.await_VimEnter()
       once = true,
       nested = true,
       callback = vim.schedule_wrap(function()
-        coroutine.resume(co)
+        assert(coroutine.resume(co))
       end),
     })