Add test that backtrace method exists on nightly

Cargo.toml

@@ -14,4 +14,5 @@ default = ["std"]
 std = []
 
 [dev-dependencies]
+rustversion = "1.0"
 thiserror = "1.0"

tests/test_backtrace.rs

@@ -0,0 +1,13 @@
+#[rustversion::not(nightly)]
+#[ignore]
+#[test]
+fn test_backtrace() {}
+
+#[rustversion::nightly]
+#[test]
+fn test_backtrace() {
+    use anyhow::anyhow;
+
+    let error = anyhow!("oh no!");
+    let _ = error.backtrace();
+}