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 <c9f5686ee5bd561c93f9de79681eee75bfa7f36e@arm.com>
Reviewed-by: Jack Travaglini <8eef13a4f970e2e9372c8d26ad0737b760f99c02@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/6623
Reviewed-by: Gabe Black <5c8aa1266c03186393d9e7b3b4e3b14239af8201@google.com>

gem5/gem5,gem5/gem5,TUD-OS/gem5-dtu,gem5/gem5,TUD-OS/gem5-dtu,gem5/gem5,TUD-OS/gem5-dtu,TUD-OS/gem5-dtu,TUD-OS/gem5-dtu,gem5/gem5,TUD-OS/gem5-dtu,gem5/gem5,gem5/gem5,TUD-OS/gem5-dtu