meson: fix testplan building for old meson

Old meson 0.47.2 do not support dictionary adding so fix
testplan building.

Cc: Mauro Carvalho Chehab <mauro.chehab@linux.intel.com>
Signed-off-by: Kamil Konieczny <kamil.konieczny@linux.intel.com>
Acked-by: Petri Latvala <adrinael@adrinael.net>