Add Config,RunningInsideUnitTest

Sometimes it is necessary for some functionality to be disabled while
running in unit tests, e.g. functionality that requires external
information such as error prone tools and configuration. Sometimes it
is necessary for some functionality to be enabled while running in
unit tests, e.g. functionality that makes state available for
testing but which is not necessary at runtime.

Previously, that was done by checking to see if TestProductVariables
was nil. This change adds a method to abstract that.

Bug: 245956352
Test: m nothing
Change-Id: I7845b79328e7180623161a9bf897568089da4e4f