Fix a couple of flaky tests in CI (#688)

Author: whatyouhide

test/sentry/plug_context_test.exs

@@ -1,5 +1,5 @@
 defmodule Sentry.PlugContextTest do
-  use Sentry.Case, async: true
+  use Sentry.Case, async: false
   use Plug.Test
 
   setup do

test/sentry/test_test.exs

@@ -168,7 +168,7 @@ defmodule Sentry.TestTest do
     assert {:ok, ""} = Sentry.capture_message("Oops from parent process")
 
     send(child_pid, :go)
-    assert_receive {:DOWN, ^monitor_ref, _, _, :normal}, 1000
+    assert_receive {:DOWN, ^monitor_ref, _, _, :normal}, 5000
     assert_receive {:done, {:ok, "340"}}, 1000
 
     assert [%Event{} = event] = Test.pop_sentry_reports()