tests: drivers: display: add read() and write() test

Add test for `display_read()` and `display_write()` api.

Note: The CI environment has no display device, which makes it fail
the tests. So, I make this test case `build_only`. But it can run in
a display device available environment.

Signed-off-by: TOKITA Hiroshi <tokita.hiroshi@gmail.com>