examples: add unit testing example

examples/system/unit_test/components/testable/include/testable.h

@@ -0,0 +1,18 @@
+/* testable.h: Implementation of a testable component.
+
+   This example code is in the Public Domain (or CC0 licensed, at your option.)
+
+   Unless required by applicable law or agreed to in writing, this
+   software is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+   CONDITIONS OF ANY KIND, either express or implied.
+*/
+
+#pragma once
+
+/**
+ * @brief Calculate arithmetic mean of integer values
+ * @param values  array of values
+ * @param count   number of elements in the array
+ * @return arithmetic mean of values, or zero count is zero
+ */
+int testable_mean(const int* values, int count);