Click Run a test,
select the Instrumentation test type and drag
the following two files:

<flutter_project_directory>/build/app/outputs/apk/debug/<file>.apk

<flutter_project_directory>/build/app/outputs/apk/androidTest/debug/<file>.apk

If a failure occurs,
you can view the output by selecting the red icon:

Uploading an Android APK from the command line

See the Firebase Test Lab section of the README
for instructions on uploading the APKs from the command line.

Uploading Xcode tests

See the Firebase TestLab iOS instructions
for details on how to upload the .zip file
to the Firebase TestLab section of the Firebase Console.

Uploading Xcode tests from the command line

See the iOS Device Testing section in the README
for instructions on how to upload the .zip file
from the command line.