scons, tests: Add support for GTest XML generation

scons, tests: Add support for GTest XML generation

The GTest framework supports result generation in XML (JUnit). Enable
this by creating unit test specific targets in the
build/${BUILD_OPTS}/unittests.${VARIANT} directory. Targets in the
directory use the following naming convention:

${SRC_PATH}/${TEST_NAME}.xml

For example, the opt version of the bitunion test built for ARM would
have this path:

build/ARM/unittests.opt/base/bituniontest.xml

Change-Id: I174dff16817734db05b08ce1d5bcf52e8697bbac
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Jack Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/6624
Reviewed-by: Gabe Black <gabeblack@google.com>