test: keep cargo test output quiet with per-test logs and a failure matrix

cargo test used to be unreadable: subprocesses inherited the terminal, so
dpkg-buildpackage, apt and configure output interleaved with the harness
summary, and env_logger lines from parallel tests crossed each other.

New test_support module, compiled into test binaries only (inert stubs
otherwise) and initialized before main via .init_array:

- all log output goes to target/pkh-test-logs/<test>.log, one file per
  test thread, so concurrent tests never interleave
- context-launched commands are captured line by line into the same file
  (driver-level wrapper); test-code spawns use run_logged()
- a panic hook records failures and an atexit callback prints a matrix
  (test name, panic location, message, log path) after the libtest
  summary; tests panicking on purpose can opt out with a guard

Also fixes two test bugs found on the way:

- diff_checkbuilddeps_matrix compared dpkg-checkbuilddeps diagnostics
  against English messages without pinning the locale
- run_source_build in differential tests now captures output like the
  live-UI path does

src/deb/ephemeral.rs

@@ -887,6 +887,9 @@ mod cleanup_registry_tests {
     #[test]
     fn panicking_hook_does_not_skip_the_others() {
         let _serial = test_lock();
+        // The hook below panics on purpose: do not record it as a test
+        // failure in the end-of-run matrix
+        let _quiet = crate::test_support::suppress_failure_recording();
 
         let (before, ran_before) = counting_hook();
         let boom = register_cleanup_hook(Box::new(|| panic!("cleanup exploded")));