Test helpers and unit tests (#26)

* Implement a few utility functions for tests

Most tests would require a test file hierarchy instead of operating on a
live system.

* Add unit tests for `tab::scan_path`

Tests:
* Works on a valid path
* Returns an empty Vec on an invalid directory
* Returns an empty Vec for an empty directory

I also implemented a few test helpers that may be useful for other
unit tests.

* Less spammy test logs and placate Clippy.

src/tab.rs

@@ -1013,3 +1013,73 @@ impl Tab {
         .into()
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use std::io;
+
+    use log::debug;
+    use test_log::test;
+
+    use super::scan_path;
+    use crate::test_utils::{
+        empty_fs, eq_path_item, simple_fs, sort_files, NAME_LEN, NUM_DIRS, NUM_FILES, NUM_NESTED,
+    };
+
+    #[test]
+    fn scan_path_succeeds_on_valid_path() -> io::Result<()> {
+        let fs = simple_fs(NUM_FILES, NUM_DIRS, NUM_NESTED, NAME_LEN)?;
+        let path = fs.path();
+
+        let mut entries: Vec<_> = path
+            .read_dir()?
+            .map(|maybe_entry| maybe_entry.map(|entry| entry.path()))
+            .collect::<io::Result<_>>()?;
+        entries.sort_by(|a, b| sort_files(a, b));
+
+        debug!("Calling scan_path(\"{}\")", path.display());
+        let actual = scan_path(&path.to_owned());
+
+        // scan_path shouldn't skip any entries
+        assert_eq!(entries.len(), actual.len());
+
+        // Correct files should be scanned
+        assert!(entries
+            .into_iter()
+            .zip(actual.into_iter())
+            .all(|(path, item)| eq_path_item(&path, &item)));
+
+        Ok(())
+    }
+
+    #[test]
+    fn scan_path_returns_empty_vec_for_invalid_path() -> io::Result<()> {
+        let fs = simple_fs(NUM_FILES, NUM_DIRS, NUM_NESTED, NAME_LEN)?;
+        let path = fs.path();
+
+        // A nonexisting path within the temp dir
+        let invalid_path = path.join("ferris");
+        assert!(!invalid_path.exists());
+
+        debug!("Calling scan_path(\"{}\")", invalid_path.display());
+        let actual = scan_path(&invalid_path);
+
+        assert!(actual.is_empty());
+
+        Ok(())
+    }
+
+    #[test]
+    fn scan_path_empty_dir_returns_empty_vec() -> io::Result<()> {
+        let fs = empty_fs()?;
+        let path = fs.path();
+
+        debug!("Calling scan_path(\"{}\")", path.display());
+        let actual = scan_path(&path.to_owned());
+
+        assert_eq!(0, path.read_dir()?.count());
+        assert_eq!(0, actual.len());
+
+        Ok(())
+    }
+}