Do not include 'tests' module in default image

I assume this was required for `make check` (without image=) to work.
Let's have 'make check' use 'tests' image by default unless overwritten.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>