Introduce control into root build scripts to disable building tests

[mmitche]

The Unified Build Controls epic changes the default for building tests when just building a normal VMR build.. Instead, tests are built. A build option should be added that will skip tests.

Source only builds have the same defaults.

[mmitche]

All the supporting functionality hasn't been added yet but the switch can be added prior to that.

[tkapin]

Depends on Viktor's work on source-build defaults.