ext: test: Split up the GTEST_CPPFLAGS and CPPFLAGS.

ext: test: Split up the GTEST_CPPFLAGS and CPPFLAGS.

scons seems to get confused in some situations when this is a single
large string and passes it as one big argument to g++ instead of
breaking it up into several arguments.

We need to do the work for it and break it into individual arguments,
like what was already being done with GTEST_LIBS.

Also wrap some overly long lines.

Change-Id: Ib7688a7abced43a9c62994d17b78d358fc0dc000
Reviewed-on: https://gem5-review.googlesource.com/c/16567
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>