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/lib.rs

@@ -37,6 +37,10 @@ pub mod ui;
 /// Handle context for .deb building: locally, over ssh, in a chroot...
 pub mod context;
 
+/// Quiet test runs: per-test log files, subprocess capture and failure
+/// matrix (inert passthrough outside test binaries)
+pub(crate) mod test_support;
+
 /// Utility functions
 pub(crate) mod utils;