tests: boards: nrf: i2c: i2c_slave: Move tests DTS files

Move DTS overlay files that modify test configuration
from boards sub-directory to the main application directory.
There is no 'i2c_speed_fast' board.

Signed-off-by: Sebastian Głąb <[email protected]>

Author: nordic-segl

tests/boards/nrf/i2c/i2c_slave/boards/i2c_speed_fast.overlay renamed to tests/boards/nrf/i2c/i2c_slave/i2c_speed_fast.overlay

@@ -41,7 +41,7 @@ tests:
       - nrf54l15dk/nrf54l15/cpuapp
       - nrf54lm20dk/nrf54lm20a/cpuapp
     extra_args:
-      - EXTRA_DTC_OVERLAY_FILE="boards/i2c_speed_fast.overlay"
+      - EXTRA_DTC_OVERLAY_FILE="i2c_speed_fast.overlay"
   boards.nrf.i2c.i2c_slave.fast_plus:
     platform_allow:
       - nrf5340dk/nrf5340/cpuapp
@@ -52,4 +52,4 @@ tests:
       - nrf54h20dk/nrf54h20/cpuapp
       - nrf54h20dk/nrf54h20/cpuppr
     extra_args:
-      - EXTRA_DTC_OVERLAY_FILE="boards/i2c_speed_fast_plus.overlay"
+      - EXTRA_DTC_OVERLAY_FILE="i2c_speed_fast_plus.overlay"