tests: build_all: gpio: Excluding `app.overlay` from ads1145s0x tests

- Exclude `app.overlay` from DTC_OVERLAY_FILES in
  `drivers.gpio.build.adc_ads1145s0x_gpio` tests to avoid duplicate builds.
- Modified so that adc_ads1145s0x_gpio.overlay can be tested alone.
- Delete the conffile and move its contents to extra_configs.

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