Steps to reproduce:

1. Build a clean local engine ios_debug_sim_unopt: autoninja -C out/ios_debug_sim_unopt
2. Run ios test with flutter/testing/run_tests.py --type=objc
3. See error message such as:

Testing failed:
	/Users/ychris/flutter/engine/src/out/ios_debug_sim_unopt/libocmock_shared.dylib: No such file or directory
	/Users/ychris/flutter/engine/src/out/ios_debug_sim_unopt/libios_test_flutter.dylib: No such file or directory
	Testing cancelled because the build failed.

This error message is not clear for what the user should do next

Expected:
The run_tests.py should log an error message to ask user to run autoninja -C out/ios_debug_sim_unopt ios_test_flutter if the ios_test_flutter artifacts aren't present. The fix can go somewhere in this function: https://github.com/flutter/engine/blob/541779d57e15d360a3c977d1dd18c3543c163d93/testing/run_tests.py#L297