Add a simple expect invocation to the test suite

I'm modifying this to check that `UPDATE_EXPECT` works manually.

src/lib.rs

@@ -724,6 +724,11 @@ fn format_chunks(chunks: Vec<dissimilar::Chunk>) -> String {
 mod tests {
     use super::*;
 
+    #[test]
+    fn test_trivial_assert() {
+        expect!["5"].assert_eq("5");
+    }
+
     #[test]
     fn test_format_patch() {
         let patch = format_patch(None, "hello\nworld\n");