Prefix googletest binaries under its own subtree instead of gtest

Building all test binaries under their respective subtrees makes
building the project via cmake easier to grok without additional hacks.
In particular, when dealing with the conversion I proposed in
https://reviews.freebsd.org/D19430 (switching from autotools to cmake),
I ran into unexpected gtest prefixing under the googlemock directory, as
opposed to the googletest directory. Example:

Before: googlemock/gtest/googletest-break-on-failure-unittest_
After: googletest/googletest-break-on-failure-unittest_

The latter form is easier to translate to packaging manifests when
building googlemock is disabled, as well as enabled, as the path remains
consistent when the feature flag is disabled.

Signed-off-by: Enji Cooper <yaneurabeya@gmail.com>