test: stop building tons of copies of gtest/gmock objects

The current makefile ends up building ~17 copies of the gtest/gmock
objects -- every test that refers to the cc files directly will have
its own copy.  This is because the build doesn't know if CFLAGS and
such have changed between each target (and in some cases, they are).

Create a new libtesting.a target to hold a single copy of these files
and update all of the unittests to link that in.  This speeds up the
build a bit especially when you aren't using ccache.

This does mean we can no longer build gtest/gmock with unique flags,
but we haven't wanted that so far, so clearly no one wants that.

BUG=chromium:579384
TEST=`make check` passes
R=thestig@chromium.org

Review URL: https://codereview.chromium.org/1633903002 .

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