Fix race condition in Tesla.Mock.agent_set

[tomekowal]

Hi!
I've run into an issue while running tests:

     ** (RuntimeError) failed to start child with the spec %{id: Tesla.Mock, start: {Agent, :start_link, [#Function<1.5545190/0 in Tesla.Mock.agent_set/1>, [name: Tesla.Mock]]}}.
     Reason: bad child specification, got: {:already_started, #PID<0.8504.0>}
     code: {:ok, activity} = create_successful_payment(%{"barcode" => tok.barcode}, acq, @payment_method)
     stacktrace:
       (ex_unit 1.18.2) lib/ex_unit/callbacks.ex:590: ExUnit.Callbacks.start_supervised!/2
       (tesla 1.14.0) lib/tesla/mock.ex:152: Tesla.Mock.mock_global/1

Quick debugging session showed that it is a race condition in the agent_set function.

It performed:

case Process.whereis(__MODULE__) do
      nil ->
        # here another process already started the mock agent
        # so the next line failed
        ExUnit.Callbacks.start_supervised!(...)

My fix removes the check and tries to start the agent immediately.

• it uses start_supervised (without !) to not crash on error
• handles the {error, {:already_started, pid}} case
• in other cases it throws an error to preserve existing behaviour.

I checked if similar pattern is used elsewhere, but the only other usage of Process.whereis is in agent_get which is invoked later and doesn't start anything.