lib/igt_core: Add support for subtest descriptions

This patch adds igt_description() which attaches a description to the
following igt_subtest or igt_subtest_group block.

Descriptions are accessible via './test --describe[=pattern]'

Subtest description is its own igt_describe as well as igt_describes
of all the parenting igt_subtest_groups, starting from the outermost
scope.

Examples of code and produced outputs are included in
lib/test/igt_describe.c and as a documentation comment on igt_describe()
macro.

v2: address Simon's review

Cc: Simon Ser <simon.ser@intel.com>
Cc: Petri Latvala <petri.latvala@intel.com>
Signed-off-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Acked-by: Petri Latvala <petri.latvala@intel.com>
Reviewed-by: Simon Ser <simon.ser@intel.com>