runner: Normalize testlist entries that don't list subtests

As the syntax for "all subtests" and "test that doesn't have subtests"
is the same, check the subtest listing when building the execution
plan. Doing that makes it possible to still have "all subtests" in
testlists (albeit not originally designed to support that) and enables
blocklisting particular subtests.

Signed-off-by: Petri Latvala <adrinael@adrinael.net>
Reviewed-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: Arkadiusz Hiler <arek@hiler.eu>
Cc: Kamil Konieczny <kamil.konieczny@linux.intel.com>
Cc: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Cc: Bhanuprakash Modem <bhanuprakash.modem@intel.com>
Cc: Ashutosh Dixit <ashutosh.dixit@intel.com>
Closes: #149