tests: test_functional: update automount log string

Update `wait_for_str` to match the new log message format for storage
automount success: `"Automount /att_storage succeeded"` replaces the
old `"/att_storage automounted"` string.

Signed-off-by: Simen S. Røstad <simen.rostad@nordicsemi.no>

Author: simensrostad

tests/on_target/tests/test_functional/test_buffer_flash.py

@@ -70,7 +70,7 @@ def test_buffer_flash(dut_cloud, hex_file_buffer_flash):
         reset_device()
 
         # Wait for storage automount
-        dut_cloud.uart.wait_for_str("/att_storage automounted", timeout=60)
+        dut_cloud.uart.wait_for_str("Automount /att_storage succeeded", timeout=60)
 
         # Clear buffer
         dut_cloud.uart.write(clear_str)