Increase error test coverage.

error.rs coverage:
	Lines: 84.26% -> 91.88%
	Functions: 37.1% -> 39.07%
Total coverage:
	Lines: 83.55% -> 87.5%
	Functions: 45.21% -> 46.42%

tests/test_context.rs

@@ -157,3 +157,10 @@ fn test_unsuccessful_downcast() {
     drop(err);
     assert!(dropped.all());
 }
+
+#[test]
+fn test_root_cause() {
+    let (err, _) = make_chain();
+
+    assert_eq!(err.root_cause().to_string(), "no such file or directory");
+}

tests/test_convert.rs

@@ -8,6 +8,26 @@ use std::error::Error as StdError;
 
 #[test]
 fn test_convert() {
+    let has_dropped = Flag::new();
+    let error = Error::new(DetectDrop::new(&has_dropped));
+    let box_dyn = Box::<dyn StdError>::from(error);
+    assert_eq!("oh no!", box_dyn.to_string());
+    drop(box_dyn);
+    assert!(has_dropped.get());
+}
+
+#[test]
+fn test_convert_send() {
+    let has_dropped = Flag::new();
+    let error = Error::new(DetectDrop::new(&has_dropped));
+    let box_dyn = Box::<dyn StdError + Send>::from(error);
+    assert_eq!("oh no!", box_dyn.to_string());
+    drop(box_dyn);
+    assert!(has_dropped.get());
+}
+
+#[test]
+fn test_convert_send_sync() {
     let has_dropped = Flag::new();
     let error = Error::new(DetectDrop::new(&has_dropped));
     let box_dyn = Box::<dyn StdError + Send + Sync>::from(error);

tests/test_downcast.rs

@@ -84,6 +84,15 @@ fn test_drop() {
     assert!(has_dropped.get());
 }
 
+#[test]
+fn test_as_ref() {
+    let error = bail_error().unwrap_err();
+    let ref_dyn = AsRef::<dyn StdError>::as_ref(&error);
+    assert_eq!("oh no!", ref_dyn.to_string());
+    let ref_dyn_send_sync = AsRef::<dyn StdError + Send + Sync>::as_ref(&error);
+    assert_eq!("oh no!", ref_dyn_send_sync.to_string());
+}
+
 #[test]
 fn test_large_alignment() {
     #[repr(align(64))]