Fix `mix test` on Elixir 1.18

[unflxw]

On Elixir 1.18, running mix test will now run the extension installation Mix task. However, it will attempt to run it in test mode, with our test configuration, which points at fake modules. Those fake modules expect their corresponding processes to have been started by their ExUnit test, but they haven't been, so it crashes:

> mix test
** (exit) exited in: GenServer.call(FakeSystem, {:get, #Function<0.38303420/1 in FakeSystem.get/2>}, 5000)
    ** (EXIT) no process: the process is not alive or there's no process currently associated with the given name, possibly because its application isn't started
    (elixir 1.18.2) lib/gen_server.ex:1121: GenServer.call/3
    test/support/fake_system.ex:5: FakeSystem.cmd/3
    mix_helpers.exs:739: Mix.Appsignal.Helper.uid/0
    mix_helpers.exs:734: Mix.Appsignal.Helper.root?/0
    mix_helpers.exs:550: Mix.Appsignal.Helper.initial_report/0
    mix_helpers.exs:25: Mix.Appsignal.Helper.install/0
    mix.exs:10: Mix.Tasks.Compile.Appsignal.run/1
    (mix 1.18.2) lib/mix/task.ex:495: anonymous fn/3 in Mix.Task.run_task/5

See #992 for a workaround, which is not a proper fix. See also Slack conversation for context.

Reproduce

• Upgrade to Elixir 1.18
• Run mix test (this will work if mix compile hasn't been ran yet, as it will cause mix test to run the installation Mix task in non-test mode)
• Run mix test again, which will attempt to run the installation Mix task in test mode and fail.

[tombruijn]

I have the issue now also on Elixir 1.17.3 and OTP 26.2.5.11