scons: Add in a new type of unit test called GTest.

scons: Add in a new type of unit test called GTest.

This type of unit test is almost the same as UnitTest, except that
it builds in the googletest library with the appropriate flags, and
it also omits all the gem5 object files by default.

Change-Id: I230db7ede563a73a9da0ebeed44aca389f724465
Reviewed-on: https://gem5-review.googlesource.com/6084
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>