ktest: Add a --dry-run mode

When working on a ktest configuration, it is often useful to inspect the
final option values after includes, defaults, per-test overrides, and
variable expansion have been applied, without actually starting a test run.

Add a --dry-run option that reads the configuration, prints the test
preamble using resolved option values, and exits before opening LOG_FILE or
executing any test logic.

This is useful for debugging ktest configurations and for scripts that need
to validate the final resolved settings without triggering side effects.

Cc: John Hawley <warthog9@eaglescrag.net>
Cc: Andrea Righi <arighi@nvidia.com>
Cc: Marcos Paulo de Souza <mpdesouza@suse.com>
Cc: Matthieu Baerts <matttbe@kernel.org>
Cc: Fernando Fernandez Mancera <fmancera@suse.de>
Cc: Pedro Falcato <pfalcato@suse.de>
Link: https://patch.msgid.link/20260307-ktest-fixes-v1-9-565d412f4925@suse.com
Signed-off-by: Ricardo B. Marlière <rbm@suse.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>