tests: scons: Integrate the googletest library into the build.

tests: scons: Integrate the googletest library into the build.

A static library is built using gtest-all.cc and includes the gtest
main function from gtest_main.cc. extra libraries to use when linking
are included in the main environment variable GTEST_LIBS, and extra
CPPFLAGS are in GTEST_CPPFLAGS. The library's path is added to the
library search path, and it's headers to the header search path.

Change-Id: I2bd6f705bd9fac5db9280996f574b168c11ff8ce
Reviewed-on: https://gem5-review.googlesource.com/6083
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>