selftest/SStatePrintdiff: ensure all base signatures are present in sstate in test_im...

selftest/SStatePrintdiff: ensure all base signatures are present in sstate in test_image_minimal_vs_base_do_configure

The test relies on all tasks in the dependency tree of the tasks being changed
having valid signatures in sstate, so that the recursive discovery of the
base invalid tasks stops there, and doesn't go further.

This may not always occur, particularly when hash equivalency combined with
different build host architectures prevents them from getting created in regular builds:
https://autobuilder.yoctoproject.org/typhoon/#/builders/127/builds/2725/steps/15/logs/stdio

The other two tests (that change specific recipes) already ensure this, but
this test (which changes a basic task definition) does not.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>