Parse debian/tests/control into runnable test descriptions: one test
per Tests: script and per Test-Command value, carrying the shared
Depends/Restrictions/Tests-Directory stanza fields and the autopkgtest
command1 enumeration (test-name overrides advance the counter).
Depends substitution is deliberately left raw: expanding @ and friends
needs the built binary list, so it belongs to the runner.
First piece of pkh test (see plans/pkh-test.md); the runner expands
these stanzas against the deb822 primitives.