[flutter_engine] Use flag to skip engine version check in test runner

The test runner in flutter/dev/bots/test.dart has an engine version
check which must be skipped on the 3H builder, because that builder
uses an engine with a different version.

This check was based on the swarming bot name. This CL prepares to use
an environment variable instead, making it independent of the swarming
bot it runs on. The change to check for this flag will be added to the
flutter/flutter repository (github.com/flutter/flutter/pull/73560).

Change-Id: Ia5c8c947be2284936f63d15e03119c4c6a134efe
Reviewed-on: https://dart-review.googlesource.com/c/recipes/+/178781
Commit-Queue: Karl Klose <karlklose@google.com>
Reviewed-by: Alexander Thomas <athom@google.com>
5 files changed