chore: Add some basic encoder tests (#2845)

Adds some encoder tests for rotating, and basic layers.
Mock Kscans don't seem to interact nicely, slightly on the hacky side.
However, better to have hacky tests than no tests.

app/tests/encoders/behavior_keymap.dtsi

@@ -0,0 +1,44 @@
+#include <dt-bindings/zmk/keys.h>
+#include <behaviors.dtsi>
+#include <dt-bindings/zmk/kscan_mock.h>
+
+/ {
+    keymap {
+        compatible = "zmk,keymap";
+
+        default_layer {
+            bindings = <
+                &tog 2 &kp X
+                &tog 1 &none
+            >;
+            sensor-bindings = <&inc_dec_kp A B>;
+        };
+        skip_layer {
+            bindings = <
+                &trans &kp Y
+                &none &none
+            >;
+            sensor-bindings = <&inc_dec_kp N M>;
+        };
+        alt_layer {
+            bindings = <
+                &trans &kp Z
+                &none &none
+            >;
+            sensor-bindings = <&inc_dec_kp C D>;
+        };
+    };
+
+    mock_encoder: mock_encoder {
+        compatible = "zmk,sensor-encoder-mock";
+        status = "okay";
+        event-startup-delay = <200>;
+        event-period = <200>;
+    };
+
+    sensors: sensors {
+        compatible = "zmk,keymap-sensors";
+        sensors = <&mock_encoder>;
+        triggers-per-rotation = <20>;
+    };
+};