t5520: factor out common "failing autostash" code

Three tests contains repetitive lines of code.

Factor out common code into test_pull_autostash_fail() and then call it in
these tests.

Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Mehul Jain <mehul.jain2029@gmail.com>
Reviewed-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>