tests: Improve the unit test framework integration with automake

tests: Improve the unit test framework integration with automake

Prior to this commit, the unit test framework behaved slightly
differently depending upon how the libcgroup git repo was
cloned.  This commit standardizes the unit test framework's
behavior by initializing the googletest submodule when
bootstrap.sh is invoked.

This commit also cleans up a potential issue on some versions
of automake that don't properly expand $(top_builddir) early
in the automake process.

Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>
Signed-off-by: Dhaval Giani <dhaval.giani@oracle.com>