refactor(tests): Move to native_posix_64 target.

* Allows removing multilib from docker images
* Run properly in aarch64 host docker
  containers for testing on Rasberry Pi.
* Small sticky-keys fix to initialize w/ correct
  constant for max uin32_t value.

app/boards/native_posix_64.overlay

@@ -0,0 +1,18 @@
+#include <behaviors.dtsi>
+#include <dt-bindings/zmk/keys.h>
+#include <dt-bindings/zmk/kscan_mock.h>
+
+/ {
+	chosen {
+		zmk,kscan = &kscan;
+	};
+
+	kscan: kscan {
+		compatible = "zmk,kscan-mock";
+		label = "KSCAN_MOCK";
+
+		rows = <2>;
+		columns = <2>;
+		exit-after;
+	};
+};