scons: Make sure GTests have the right environment variables

scons: Make sure GTests have the right environment variables

SCons currently scrubs the environment variables used by GTests too
aggressively. This breaks systems where libraries are installed in
non-standard locations that need to be specified in
LD_LIBRARY_PATH. Run said tests in the gtest_env SCons environment
which white-lists the important environment variables.

Change-Id: I5fc8fb5e51f09644dc976ee97b21c78ab349bf7d
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Jack Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/6623
Reviewed-by: Gabe Black <gabeblack@google.com>