[functional-tests] port some of the cache_check tests to Rust

tests/common/test_dir.rs

@@ -0,0 +1,27 @@
+use anyhow::Result;
+use std::path::{PathBuf};
+use tempfile::{tempdir, TempDir};
+
+//---------------------------------------
+
+pub struct TestDir {
+    dir: TempDir,
+    file_count: usize,
+}
+
+impl TestDir {
+    pub fn new() -> Result<TestDir> {
+        let dir = tempdir()?;
+        Ok(TestDir { dir, file_count: 0 })
+    }
+
+    pub fn mk_path(&mut self, file: &str) -> PathBuf {
+        let mut p = PathBuf::new();
+        p.push(&self.dir);
+        p.push(PathBuf::from(format!("{:02}_{}", self.file_count, file)));
+        self.file_count += 1;
+        p
+    }
+}
+
+//---------------------------------------