Add some dummy tests for Rust examples

Author: szg251

examples/rust-flake-project-cross-compilation/tests/test.rs

@@ -0,0 +1,7 @@
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn test() {
+        assert_eq!(1 + 1, 2);
+    }
+}

examples/rust-flake-project-with-extra-dependency/tests/test.rs

@@ -0,0 +1,7 @@
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn test() {
+        assert_eq!(1 + 1, 2);
+    }
+}

examples/rust-flake-project/tests/test.rs

@@ -0,0 +1,7 @@
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn test() {
+        assert_eq!(1 + 1, 2);
+    }
+}