Tackle flakey iex warning test

Sometimes the iex(2)> (at least on CI) is sent together with the
output of the benchmark/the benchmark suite. That caused it to
be flakey.

This change should accommodate for that and lead to less flakey
results.

Author: PragTob

test/benchee_test.exs

@@ -1057,14 +1057,29 @@ defmodule BencheeTest do
         assert_receive {^port, {:data, "Warning: " <> message}}, 20_000
         assert message =~ ~r/test.+evaluated.+slower.+compiled.+module.+/is
 
-        # waiting for iex to be ready for input again
-        assert_receive {^port, {:data, "iex(2)> "}}, 20_000
+        # waiting for iex to be ready for input again/the benchmark to be finished
+        # sometimes we get "iex(2)>" as a separate message, sometimes it's attached to the
+        # previous output - hence we gotta doe out own `receive` checking.
+        assert :ok = wait_for_benchmark_finished(port)
       after
         # https://elixirforum.com/t/starting-shutting-down-iex-with-a-port-gracefully/60388/2?u=pragtob
         send(port, {self(), {:command, "\a"}})
         send(port, {self(), {:command, "q\n"}})
       end
     end
+
+    defp wait_for_benchmark_finished(port) do
+      receive do
+        {^port, {:data, output}} ->
+          if String.contains?(output, "iex(2)>") do
+            :ok
+          else
+            wait_for_benchmark_finished(port)
+          end
+      after
+        20_000 -> raise RuntimeError, "Waited too long for iex benchmark to finish/send iex(2)>"
+      end
+    end
   end
 
   describe "escript building" do