tests: Split overlay tests into those with do/don't exercise dtc plugin generation

The various tests for overlay/plugin support are currently lumped together
in the overlay_tests shell function, which is executed by libfdt_tests.

However, this includes both tests designed primarily to exercise libfdt's
overlay application, and tests designed to exercise dtc's overlay
generation.  Split these up for improved clarity.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>