wgsl-errors: Reverse "old" and "new" sides of diff output.

When an error snapshot test fails and we generate a diff comparing the
expected output with the actual output, treat the expected output as
the diff "from", and the actual output as the diff "to" - not the
reverse.

tests/wgsl-errors.rs

@@ -8,7 +8,7 @@ fn check(input: &str, snapshot: &str) {
  .expect_err("expected parser error")
  .emit_to_string(input);
  if output != snapshot {
- for diff in diff::lines(&output, snapshot) {
+ for diff in diff::lines(snapshot, &output) {
  match diff {
  diff::Result::Left(l) => println!("-{l}"),
  diff::Result::Both(l, _) => println!(" {l}"),