Testing: split ble encoder test (#2754)

test(sensors): Add mock encoder sensor

Add a new mock sensor driver for generating mock encoder sensor channel
data.

test(split): Add a peripheral sensor split test

Test BLE split logic for peripheral sensors (encoders).

app/tests/ble/split/peripheral-encoder/shared.dtsi

@@ -0,0 +1,12 @@
+/ {
+    mock_encoder: mock_encoder {
+        compatible = "zmk,sensor-encoder-mock";
+        status = "disabled";
+    };
+
+    sensors: sensors {
+        compatible = "zmk,keymap-sensors";
+        sensors = <&mock_encoder>;
+        triggers-per-rotation = <20>;
+    };
+};