Add tests for ClipboardContext and NopClipboardContext

copypasta doesn't seem to have unit tests, so I accidentally made
changes with the PR: alacritty#57

I want to make sure any changes are checked by running test codes.
That's why I want to add test codes.

src/nop_clipboard.rs

@@ -39,3 +39,18 @@ impl ClipboardProvider for NopClipboardContext {
         Ok(())
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use crate::common::ClipboardProvider;
+    use crate::nop_clipboard::NopClipboardContext;
+
+    #[test]
+    fn it_does_nothing() {
+        let mut ctx = NopClipboardContext::new().unwrap();
+        let msg = "Hello, world!";
+        ctx.set_contents(msg.to_owned()).unwrap();
+
+        assert_eq!(ctx.get_contents().unwrap(), "");
+    }
+}

tests/main.rs

@@ -0,0 +1,10 @@
+use copypasta::{ClipboardContext, ClipboardProvider};
+
+#[test]
+fn it_works() {
+    let mut ctx = ClipboardContext::new().unwrap();
+    let msg = "Hello, world!";
+
+    ctx.set_contents(msg.to_owned()).unwrap();
+    assert_eq!(ctx.get_contents().unwrap(), "Hello, world!");
+}