Actually implement --dry-run to not execute tests. With dry-run
active, attempting to execute will figure out the list of things to
execute, serialize them along with settings, and stop. This will be
useful for CI that wants to post-mortem on failed test rounds to
generate a list of tests that should have been executed and produce
json result files (full of 'notrun') for proper statistics.

Signed-off-by: Petri Latvala <petri.latvala@intel.com>
Cc: Andi Shyti <andi.shyti@intel.com>
Cc: Martin Peres <martin.peres@linux.intel.com>
Cc: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Cc: Tomi Sarvela <tomi.p.sarvela@intel.com>
Reviewed-by: Martin Peres <martin.peres@linux.intel.com>