ci logs are too long for GitLab CI

git.purrdata.net has a limit of 4194304 bytes for CI logs and this is sometimes exceeded by CI jobs, e.g. https://git.purrdata.net/jwilkes/purr-data/-/jobs/18898 and https://git.purrdata.net/jwilkes/purr-data/-/jobs/18862

Here are some things to investigate to reduce the size of logs:

• Enable Automake's silent mode so compiler/linker commandlines are only shown when an error occurs
• fix the many dpkg-shlibdeps warnings that occur
• modify the tests to only show output when there's a problem (i don't know how the tests work, but maybe it's simple to do)