ART: Add env variable to increase run-test timeouts

To generally increase timeouts, allow an environmental factor. This
is meant for slow hosts or devices.

We may want to look into trying to auto-detect the necessity of this.

Bug: 140935398
Test: ART_TIME_OUT_MULTIPLIER=10 art/test/run-test --host --dev 001-HelloWorld
Change-Id: I3f751b5e824da52e6441691b944eb99c1c3692b3
1 file changed