Fix Test.expect in shared mode

[hkrutzer]

This commit fixes

Req.Test.set_req_test_to_shared()

Req.Test.expect(MyApp.Weather, fn conn ->
  Req.Test.json(conn, %{"celsius" => 25555.0})
end)

assert {:ok, %Req.Response{}} == Reqrepro.hello()

which would produce

     ** (CaseClauseError) no case clause matching: %{expectations: [#Function<0.11329399/1 in ReqreproTest."test test"/1>]}
     code: assert {:ok, %Req.Response{}} == Reqrepro.hello()
     stacktrace:
       (req 0.5.8) lib/req/test.ex:390: Req.Test.__fetch_plug__/1
       (req 0.5.8) lib/req/test.ex:632: Req.Test.call/2
       (req 0.5.8) lib/req/steps.ex:964: Req.Steps.run_plug/1
       (req 0.5.8) lib/req/request.ex:1106: Req.Request.run_request/1
       (req 0.5.8) lib/req/request.ex:1050: Req.Request.run/1

[wojtekmach]

Thank you!