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/context/mod.rs

@@ -8,6 +8,10 @@ mod ssh;
 mod unshare;
 
 pub use api::{Context, ContextCommand, ContextConfig, LineSink, Stream};
+// The driver trait is implementation detail of the context API; it is only
+// needed crate-internally (test-run capture wrapper), so keep it out of the
+// public surface (and its documentation requirement).
+pub(crate) use api::ContextDriver;
 pub use manager::ContextManager;
 use std::sync::Arc;