[librsvg: 15/23] When a test reference file doesn't exist, panic with a better message

[Federico Mena Quintero <federico src gnome org>]

commit bb1bdb6dc3d9ab56d084e462cfaeb8dbe94e31f1
Author: Federico Mena Quintero <federico gnome org>
Date:   Fri May 21 19:55:20 2021 -0500

    When a test reference file doesn't exist, panic with a better message
    
    Otherwise the plain unwrap() doesn't print what file couldn't be opened.

 tests/src/predicates/png.rs | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
---
diff --git a/tests/src/predicates/png.rs b/tests/src/predicates/png.rs
index 22767b2f..e0cf6f6c 100644
--- a/tests/src/predicates/png.rs
+++ b/tests/src/predicates/png.rs
@@ -129,7 +129,8 @@ impl ReferencePredicate<PngPredicate> {
     }
 
     fn diff_surface(&self, surface: &SharedImageSurface) -> Option<BufferDiff> {
-        let reference = Reference::from_png(&self.path).unwrap();
+        let reference = Reference::from_png(&self.path)
+            .unwrap_or_else(|_| panic!("could not open {:?}", self.path));
         if let Ok(diff) = reference.compare(&surface) {
             if !Self::diff_acceptable(&diff) {
                 return Some(diff);