resources: Fix race condition in test helper

resources: Fix race condition in test helper

The fetchResourceForSpec function was reusing a single ReadSeekCloser
across multiple goroutines, causing a race condition when
TestImageTransformConcurrent runs with imageProcWorkers > 1.

This was exposed by commit e569dd59a ("Misc webp performance work")
which increased imageProcWorkers from 1 to 2.

Fix by creating a new reader on each call to the open function.

Fixes race condition causing segfault on Apple Silicon.

Co-authored-by: Bjørn Erik Pedersen <bjorn.erik.pedersen@gmail.com>