rtla/tests: Add unit tests for CLI option callbacks

rtla/tests: Add unit tests for CLI option callbacks

In addition to testing all tool_parse_args() functions, test also all
callbacks used for parsing custom option formats.

The callbacks represent a middle layer between the parsing functions
and utility functions dedicated to checking specific argument formats,
for example, scheduling class and duration. Callback tests are run
before parsing functions to make sure any issue in the former is
reported before it is encountered through the latter.

Tests verify both successful parsing and proper rejection of invalid
inputs (via exit tests). To enable testing static callbacks, a pragma
once guard is added to timerlat.h for safe inclusion by cli_p.h.

Add dependency of UNIT_TESTS_IN on LIBSUBCMD_INCLUDES, as the new test
file tests/unit/cli_opt_callback.c includes cli_p.h which includes
subcmd/parse-options.h.

Link: https://lore.kernel.org/r/20260528103254.2990068-7-tglozar@redhat.com
Signed-off-by: Tomas Glozar <tglozar@redhat.com>