tests: drivers: build_all: display: fix conflicting nodelabels

Some nodelabels in the display build_all test overlay lacked a "test"
prefix, causing test failures on boards that also define displays with
this nodelabel. Prefix these nodes with "test" to resolve this issue.

Fixes #81610

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>