finish testing

- convert project to a library by moving program logic into lib.rs
- create ./tests to store integration tests
- create ./common to store code we want shared across integration tests

testing/tests/integration_tests.rs

@@ -0,0 +1,24 @@
+use testing::ops::*;
+
+mod common;
+
+#[test]
+fn subtract_and_divide() {
+    let a = 20;
+    let b = 2;
+    let c = 3;
+
+    let difference = sub(&a, &b);
+    let quotient = div(&difference, &c);
+    assert_eq!(quotient, 6);
+}
+#[test]
+fn add_and_multiply() {
+    let a = 10;
+    let b = common::give_me_5() as i32;
+    let c = 3;
+
+    let sum = add(&a, &b);
+    let product = mul(&sum, &c);
+    assert_eq!(product, 45);
+}