tests: drivers: display: controlling test execution with fixture

Currently, the `display_read_write` test execution can be suppressed
by `harness: display`, but since the display harness does not exist,
there is no way to run tests.
Using a fixture, it will be possible only to build if not specified,
and run tests if specified.

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