pkg-cmake.mk: globally disable BUILD_TESTING flag

This CMake flag is used to enable tests. It may not disable the test
programs from being built, but it controls the test execution.

Since we don't care about building the tests (and usually disable them
when possible), make sure Buildroot won't try to run them.

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Cc: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>