Add verbose engine logs for iOS start up tests (#130511)

Added to be able to view logs from https://github.com/flutter/engine/pull/43616.

Added to debug https://github.com/flutter/flutter/issues/129836

dev/devicelab/lib/tasks/perf_tests.dart

@@ -786,6 +786,9 @@ class StartupTest {
           '--verbose',
           '--profile',
           '--trace-startup',
+          // TODO(vashworth): Remove once done debugging https://github.com/flutter/flutter/issues/129836
+          if (device is IosDevice)
+            '--verbose-system-logs',
           '--target=$target',
           '-d',
           device.deviceId,