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/peripheral.overlay

@@ -0,0 +1,18 @@
+
+#include <dt-bindings/zmk/kscan_mock.h>
+
+#include "shared.dtsi"
+
+&kscan {
+    events = <>;
+
+    /delete-property/ exit-after;
+};
+
+&mock_encoder {
+    status = "okay";
+
+    event-startup-delay = <2000>;
+    event-period = <2000>;
+    events = <18 (-18)>;
+};