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>

Author: soburi

tests/drivers/display/display_read_write/testcase.yaml

@@ -5,7 +5,9 @@ common:
   tags:
     - drivers
     - display
-  harness: display
+  harness: ztest
+  harness_config:
+    fixture: display
 tests:
   drivers.display.read_write.sdl.argb8888:
     platform_allow: