fix: re-add the basic doc test

It got lost with the `README.md` conversion.

Signed-off-by: Harald Hoyer <harald@hoyer.xyz>

tests/test_basic.rs

@@ -0,0 +1,34 @@
+use chainerror::prelude::v1::*;
+
+#[test]
+fn test_basic() {
+    use std::path::PathBuf;
+    type BoxedError = Box<dyn std::error::Error + Send + Sync>;
+    fn read_config_file(path: PathBuf) -> Result<(), BoxedError> {
+        // do stuff, return other errors
+        let _buf = std::fs::read_to_string(&path).context(format!("Reading file: {:?}", &path))?;
+        // do stuff, return other errors
+        Ok(())
+    }
+    fn process_config_file() -> Result<(), BoxedError> {
+        // do stuff, return other errors
+        read_config_file("_non_existent.txt".into()).context("read the config file")?;
+        // do stuff, return other errors
+        Ok(())
+    }
+
+    if let Err(e) = process_config_file() {
+        let os_notfound_error = std::io::Error::from_raw_os_error(2);
+        eprintln!("Error:\n{:?}", e);
+        let s = format!("{:?}", e);
+        let lines = s.lines().collect::<Vec<_>>();
+        assert_eq!(lines.len(), 5);
+        assert!(lines[0].starts_with("tests/test_basic.rs:"));
+        assert_eq!(lines[1], "Caused by:");
+        assert!(lines[2].starts_with("tests/test_basic.rs:"));
+        assert_eq!(lines[3], "Caused by:");
+        assert_eq!(lines[4], format!("{:?}", os_notfound_error));
+    } else {
+        panic!();
+    }
+}