meson: add support for the testsuite

meson: add support for the testsuite

Currently the stale tracking of modules/rootfs is non-existent and I
cannot quite find a way to fix that.

In addition, there is a long standing meson bug where the tests and by
extension their dependencies are always build even when annotated as
`build_by_default : false`. To workaround that, a new option is
introduced `build-tests`, defaulting to false.

Apart from that, all tests run and pass \o/

v2:
 - split from top-level meson.build to subdir one
 - add to EXTRA_DIST
 - scripts are in scripts/
 - add option to enable building tests
 - error out if building without tools

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://github.com/kmod-project/kmod/pull/86
Signed-off-by: Lucas De Marchi <lucas.de.marchi@gmail.com>