sandbox: test: Show hex values on failure

Quite a few tests use addresses or hex values for comparisons. Add hex
output for test failures, e.g.:

   0x55ca22fa == reg: Expected 0x55ca22fa (1439310586),
	got 0x55ea22fb (1441407739)

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>