tests: Make overlay/plugin tests unconditional

When overlay apply supprt was added to libfdt the testcases included some
which could only be executed with the (then) out of tree dtc with overlay
output support.  So, the test script automatically skipped those tests if
it wasn't available.

Now that the overlay support is merged into dtc mainline there's no reason
to keep this logic.  Instead run all the overlay tests unconditionally.

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