Please allow overriding of @skip_unless_tools_exist failures if tools are missing
diffoscope recommends apktool but apktool is not currently in buster. The test suite of diffoscope currently passes without error because the tests that use apktool are decorated with @tool_required('apktool'). Consequently: * the code paths that use apktool are not being tested in buster * these tests are not actually passing but ci.debian.net is saying that the test suite has passed (skipping tests to hide errors isn't the same as a test passing) There is clear value in being able to skip tests for the casual user on another platform or when not interested in diffoscope's abilities in file formats about which one does not care. For the Debian package, however, I think every functionality of the Debian package should be tested in autopkgtest. The current situation does not pick up that apktool is not installable in buster, for instance, and would not pick up something like /usr/bin/apktool being renamed. Perhaps @tool_required could be extended with an "all tools required" mode such that no tests are skipped or missing tools are a test failure (or error). An environment variable would work for setting that mode and that mode used at package build time and in autopkgtest.