igt_hook: Allow multiple hook descriptors

Extend the current hook functionality to allow using multiple hook
descriptors. That allows running a test binary like the following:

    my-test --hook pre-subtest:do-something \
            --hook post-subtest:do-somthing-else

Which is more convenient to the user than having to implement a script
that checks the value of IGT_HOOK_EVENT environment variable.

Note that we still need to add the same support for igt_runner, which is
left for a followup change.

Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Gustavo Sousa <gustavo.sousa@intel.com>
Link: https://lore.kernel.org/r/20240814204822.95283-6-gustavo.sousa@intel.com


Signed-off-by: Matt Roper <matthew.d.roper@intel.com>