prune: garbage-collect pkh test run directories

Test artifacts land under /var/tmp/pkh/tests (<source>-<series>-<arch>-
<timestamp>/), one directory per run: they are disposable results, not
state worth keeping. Prune removes them on the same 7-day retention as
build sessions, and with --all; the root is overridable through
PKH_TESTS_DIR like the sessions root.

src/test/mod.rs

@@ -530,12 +530,22 @@ fn drop_shell(
 /// `/var/tmp/pkh/tests/<source>-<series>-<arch>-<timestamp>/`.
 fn create_run_dir(package: &str, series: &str, arch: &str) -> std::io::Result<PathBuf> {
     let timestamp = chrono::Utc::now().format("%Y%m%dT%H%M%S");
-    let dir =
-        PathBuf::from("/var/tmp/pkh/tests").join(format!("{package}-{series}-{arch}-{timestamp}"));
+    let dir = runs_root().join(format!("{package}-{series}-{arch}-{timestamp}"));
     fs::create_dir_all(&dir)?;
     Ok(dir)
 }
 
+/// The test-run artifacts root: `$PKH_TESTS_DIR`, defaulting to
+/// `/var/tmp/pkh/tests`.
+pub fn runs_root() -> PathBuf {
+    if let Ok(dir) = std::env::var("PKH_TESTS_DIR")
+        && !dir.is_empty()
+    {
+        return PathBuf::from(dir);
+    }
+    PathBuf::from("/var/tmp/pkh/tests")
+}
+
 /// Render the tests declared in the tree's `debian/tests/control`, one
 /// block per test (`pkh test --list-tests`).
 pub fn list_tests(tree: &Path) -> Result<String, Box<dyn std::error::Error>> {