samples: sensor: Use dedicated overlay for fake sensor in sensor_shell

Using an app.overlay file is inherently preventing the use of board
overlays. Switch to a dedicated overlay file so that e.g.
the vmu_rt1170/mimxrt1176/cm7 board target can pass this test where
before it would have failed due to its board-specific overlay "winning"
over the app.overlay file.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>