implement stack with some initial testing

test_stack.c

@@ -0,0 +1,47 @@
+#include "unity.h"
+#include "stack.h"
+
+int data1 = 10;
+int data2 = 20;
+int data3 = 30;
+
+StackNode *stack = NULL;
+
+void setUp(void)
+{
+    // set stuff up here
+}
+
+void tearDown(void)
+{
+    clearStack(stack);
+}
+
+void test_push_and_pop(void)
+{
+    stack = push(stack, &data1);
+    stack = push(stack, &data2);
+    stack = push(stack, &data3);
+
+    TEST_ASSERT_EQUAL_PTR(top(stack), &data3);
+    stack = pop(stack);
+    TEST_ASSERT_EQUAL_PTR(top(stack), &data2);
+    stack = pop(stack);
+    TEST_ASSERT_EQUAL_PTR(top(stack), &data1);
+    stack = pop(stack);
+}
+
+void test_handle_NULL(void)
+{
+    TEST_ASSERT_NULL(pop(stack));
+    TEST_ASSERT_NULL(top(stack));
+}
+
+int main(void)
+{
+    printf("============================\nStack tests\n============================\n");
+    UNITY_BEGIN();
+    RUN_TEST(test_push_and_pop);
+    RUN_TEST(test_handle_NULL);
+    return UNITY_END();
+}