test: Add logging to test/apps/signal

Signed-off-by: Lewis Marshall <lewis@lmars.net>
test: Remove docker backend reference

Signed-off-by: Lewis Marshall <lewis@lmars.net>
test: Dump logs from all booted instances

Signed-off-by: Lewis Marshall <lewis@lmars.net>
Merge pull request #582 from flynn/test-debug

Improve CI debugging