Fix: Tools: correct default open path in coverage report summary

The `/blender` prefix was specific to my local setup and may be different
for others.

tests/coverage/coverage_report/index_template.html

@@ -640,7 +640,7 @@
       let root_row = null;
 
       // Used to find which path should be opened by default if there is no other information available.
-      const fallback_default_path_segment = "/blender/source/blender";
+      const fallback_default_path_segment = "/source/blender";
 
       // Get filters from URL.
       let custom_root_paths = [];