ci: Increase default assert_receive timeout to 500ms from 100ms (#5417)

We seem to be hitting `assert_receive`-style much more frequently after
"upgrading" to Enterprise Cloud (our credits expired, I was able to
renew them).

This updates the global timeout to 500ms for `assert_receive` to reduce
the likelihood `assert_push` and friends will time out on slow GH
runners.

E.g.


https://github.com/firezone/firezone/actions/runs/9556532328/job/26341986456

---------

Signed-off-by: Jamil <jamilbk@users.noreply.github.com>

elixir/config/test.exs

@@ -73,10 +73,18 @@ config :wallaby,
   js_logger: false,
   hackney_options: [timeout: 10_000, recv_timeout: 10_000]
 
-config :ex_unit,
-  formatters: [JUnitFormatter, ExUnit.CLIFormatter],
-  capture_log: true,
-  exclude: [:acceptance]
+ex_unit_config =
+  [
+    formatters: [JUnitFormatter, ExUnit.CLIFormatter],
+    capture_log: true,
+    exclude: [:acceptance]
+  ] ++
+    case System.get_env("CI_ASSERT_RECEIVE_TIMEOUT_MS") do
+      nil -> []
+      timeout -> [assert_receive_timeout: String.to_integer(timeout)]
+    end
+
+config :ex_unit, ex_unit_config
 
 # Initialize plugs at runtime for faster development compilation
 config :phoenix, :plug_init_mode, :runtime