Add Rust unittest so that test_coverage properly self-tests

Using kcov, a coverage of 33.3% was reported, even though that are no
rust-test code in the dummy crate that is included in this repository.
This commit adds an actual unit test, so that the test_coverage test
actually properly self-tests here.

Signed-off-by: Patrick Roy <roypat@amazon.co.uk>

coverage_config_x86_64.json

@@ -1,5 +1,5 @@
 {
-    "coverage_score": 33.3, 
+    "coverage_score": 100.0, 
     "exclude_path": "", 
     "crate_features": ""
 }

src/lib.rs

@@ -1,3 +1,11 @@
-pub fn main() {
-    println!("It works!");
+pub fn add(a: u32, b: u32) -> u32 {
+    a + b
+}
+
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn test_add() {
+        assert_eq!(super::add(1, 2), 3)
+    }
 }