support/test-pkg: add option to limit the number of tests

Sometimes, it interesting to have a global overview of whether the
package builds at all or not, rather than test on all toolchains.

Add an option that allows testing on a limited set of randomly choosen
toolchains.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Luca Ceresoli <luca@lucaceresoli.net>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>