cmake: add BUILD_TESTING support
cmake: add BUILD_TESTING support

Currently tests/ requires pulling gtest from git, which is far from
ideal for distribution builds (not to mention we don't need to unit test
in an RPM build). So add the ability to disable tests/ builds.

Signed-off-by: Aleksa Sarai <b305c9b0212d6145eea67bbffd24e29cb79c759c@cyphar.com>

iovisor/bpftrace,iovisor/bpftrace,iovisor/bpftrace,iovisor/bpftrace