Make the App own the Platform

Removes optional platform, which only existed for tests.

There is now a global `@import("testing.zig").test_app` available. This is setup
when the test runner starts, and cleaned up at the end of tests. Individual
tests don't have to worry about creating app, which I assume was the reason I
Platform optional, since that woul dhave been something else that needed to be
setup.

src/telemetry/telemetry.zig

@@ -186,10 +186,7 @@ test "telemetry: getOrCreateId" {
 }
 
 test "telemetry: sends event to provider" {
-    var app = testing.createApp(.{});
-    defer app.deinit();
-
-    var telemetry = try TelemetryT(MockProvider).init(app, .serve);
+    var telemetry = try TelemetryT(MockProvider).init(testing.test_app, .serve);
     defer telemetry.deinit();
     const mock = &telemetry.provider;