lib: Introduce dynamic subsubtests

Dynamic subsubtests, or subtests of subtests, are individual pieces of
tests that are not statically available all the time.

A good example of a need for a dynamic subsubtest is i915 engine
listing: A normal subtest for each engine class ("bsd"), and a dynamic
subsubtest for each instance ("bsd0", "bsd2", etc). Or a normal
subtest for an operation with a dynamic subsubtest for every engine
there is.

Another example is dynamic subsubtests for pipes: Instead of using
foreach_pipe_static, make one subtest and use foreach_pipe with
dynamic subsubtests for each pipe.

v2: Rebase and adapt to igt_describe changes

v3: Rename to igt_subtest_with_dynamic_subsubtests &
    igt_dynamic_subsubtest, better docs, make igt_describe fail loudly
    if it's used in an impossible context.

v4: Mention dynamic blocks in the warning for longjmp in core docs.

Signed-off-by: Petri Latvala <petri.latvala@intel.com>
Reviewed-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>