added a testcase against .snap.new files existing

tests/integration_snapshots.rs

@@ -129,3 +129,53 @@ fn executable_path() -> PathBuf {
     .canonicalize()
     .expect("error canonicalizing")
 }
+
+/// lists all files in the directory recursively
+/// errors lead to panics as this is only used in a testcase
+fn recurse_list_files(path: impl AsRef<Path>) -> Vec<std::fs::DirEntry> {
+    let mut buf = vec![];
+    let entries = std::fs::read_dir(path).unwrap();
+
+    for entry in entries {
+        let entry = entry.unwrap();
+        let meta = entry.metadata().unwrap();
+
+        if meta.is_dir() {
+            let mut subdir = recurse_list_files(entry.path());
+            buf.append(&mut subdir);
+        }
+        if meta.is_file() {
+            buf.push(entry);
+        }
+    }
+
+    buf
+}
+
+/// makes sure that no `.snap.new` snapshots exist
+///
+/// This testcase exists as [`insta`] behaves as following if seeing both a having both a `.snap.new` and a `.snap` file:
+/// - the content of both files is equal, it will pass without failure and remove `.snap.new`-file
+/// - if not, it will fail and show the diff
+///
+/// This behavior is problematic as
+/// - we might not pick up on a `.snap.new` file being added as the testcase pass in CI.
+/// - future contributors would be confused where this change comes from
+/// => we are asserting that no such files exist
+#[test]
+fn no_new_snapshots() {
+    let files = recurse_list_files("test_outputs/");
+    let new_snaps=files.into_iter()
+        .map(|f| f.path())
+        .filter(|f| {
+            if f.as_path().to_str().unwrap().contains("snap.new") {
+                println!("{:?}",f.display());
+            }
+            if let Some(name)=f.file_name().unwrap().to_str() {
+                return name.ends_with("snap.new");
+            }
+            return false;
+        })
+        .collect::<Vec<_>>();
+    assert_eq!(new_snaps,Vec::<PathBuf>::new(), "`.snap.new` files exit, but should not. Did you\n- forget to run `cargo insta review` or\n- forget to move the `.snap.new` file to `.snap` after verifying the content is exactly as expected?");
+}