minor: Make union_with_mix_of_presorted_and_explicitly_resorted_inputs_impl cross platform (#22478)

## Which issue does this PR close?

- N/A.

## Rationale for this change

Fix two tests in `core` due to path issues.

## What changes are included in this PR?

- Updated
`union_with_mix_of_presorted_and_explicitly_resorted_inputs_impl` to
handle the path in a cross platform way.

## Are these changes tested?

Yes.

## Are there any user-facing changes?

No.

Author: nuno-faria

datafusion/core/tests/dataframe/mod.rs

@@ -3345,7 +3345,11 @@ async fn union_with_mix_of_presorted_and_explicitly_resorted_inputs_impl(
 
     // To be able to remove user specific paths from the plan, for stable assertions
     let testdata_clean = Path::new(&testdata).canonicalize()?.display().to_string();
-    let testdata_clean = testdata_clean.strip_prefix("/").unwrap_or(&testdata_clean);
+    let testdata_clean = testdata_clean.replace("\\", "/");
+    let testdata_clean = testdata_clean
+        .strip_prefix("//?/")
+        .or_else(|| testdata_clean.strip_prefix("/"))
+        .unwrap_or(&testdata_clean);
 
     // Use displayable() rather than explain().collect() to avoid table formatting issues. We need
     // to replace machine-specific paths with variable lengths, which breaks table alignment and