This fixes tests broken in my last change, "Merge
diff-font-lock-refine and diff-auto-refine-mode into diff-refine" from
2019-02-24.
* test/lisp/vc/diff-mode-tests.el (diff-mode-test-font-lock):
Bind diff-refine to symbol 'font-lock' instead of binding
diff-font-lock-refine to t.
(diff-mode-test-font-lock-syntax-one-line): Bind diff-refine
to nil instead of binding diff-font-lock-refine to nil.