AndroidX Webkit: add test instructions for testapp
AndroidX Webkit: add test instructions for testapp

No change to logic, only docs. This adds test instructions to the
testapp.

Test: Upload to gerrit > open file > click "gitiles"
Test: ./gradlew :webkit:integration-tests:testapp:connectedAndroidTest
Change-Id: I674de84a0bbe6ec5fa6040f40a644a296762ca1e

androidx/androidx,androidx/androidx,androidx/androidx,androidx/androidx,AndroidX/androidx,androidx/androidx,AndroidX/androidx,androidx/androidx,AndroidX/androidx,AndroidX/androidx,AndroidX/androidx,AndroidX/androidx,AndroidX/androidx,androidx/androidx,androidx/androidx,AndroidX/androidx,AndroidX/androidx,androidx/androidx,AndroidX/androidx,androidx/androidx