t3301 (notes): use test_expect_code for clarity

t3301 (notes): use test_expect_code for clarity

Use the test_expect_code helper instead of open-coding it.

The main behavior change is to print the command and actual exit
status when the test fails.  More importantly, this would make it
easier to add commands before "git notes show" as part of the
same test assertion if needed.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>