[build] Only run tests if something changed?

[Dain Sundstrom]

Can we change the build system to by default only run the module unit 
test if something changed in the module?

When we get integration tests, I would like the default to by default 
only run the integration if something changed in the domain being 
tested, but that is for the future.

-dain
/*************************
 * Dain Sundstrom
 * Partner
 * Core Developers Network
 *************************/