tests: Make "UnitTest"s more like GTest so they can be in other dirs.

tests: Make "UnitTest"s more like GTest so they can be in other dirs.

The original implementation of UnitTest forced all the output binaries
to live in the unittest directory, effectively forcing a flat
namespace, and seperating the tests from the things they were supposed
to be testing.

This changes makes them work more like the newer GTest tests in that
they can be based out of whatever directory makes sense, although
they're currently all still in unittest for the time being.

This change also gets rid of automatically tagging the sources
associated with a test with the tests name. The first reason for that
was that this also forced a flat namespace, since the tests names
didn't have any reference to the test's path. Second, this way of
pulling in additional files wasn't necessary any more, now that the
UnitTest sources could be source filters like they can be for GTests.

Change-Id: I3d96ed766ac5170842dbd6daee39f2873bcd6c75
Reviewed-on: https://gem5-review.googlesource.com/10701
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>