debian: add the DEP-8 tests-control parser

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.

src/lib.rs

@@ -15,7 +15,7 @@ pub(crate) mod data;
 /// Build a Debian package into a binary (.deb)
 pub mod deb;
 /// Reusable Debian format primitives (control/deb822, checksums, versions,
-/// changelog entries, artifact registries)
+/// changelog entries, test metadata, artifact registries)
 pub mod debian;
 /// Obtain general information about distribution, series, etc
 pub mod distro_info;