Do not add test/spec files to function path in _prepend_tree.

functions/_prepend_tree.fish

@@ -102,6 +102,11 @@ function _prepend_tree -d "Add a dependency tree to the Fish path."
       printf "%s\n" $dir
     end
 
+    # Do not add test/spec files to function path.
+    if contains -- (basename $dir) "test" "tests" "spec"
+      continue
+    end
+
     # Prepend matched directory to the the global fish function path.
     # Note path duplicates are already handled by _prepend_path.
     _prepend_path $dir -d fish_function_path