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>
run.py: introduce --dry-run option

It prints the command lines without actually running them.

--
Changes since V2:
 - changed option to --dry-run from --just-print (Glauber)
 - in addition to qemu and xen, suppress imgedit.py invocation too
 - when printing wrap arguments containing spaces with quotes

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