Fix emails helper test

Fix emails helper test

JH change the image alt from "GitLab" to "Jihu GitLab", upstream test failed, so extract the assertion value into module to let JH override it.

cc: @daveliu

JH MR: https://jihulab.com/gitlab-cn/gitlab/-/merge_requests/532

before

expect(header_logo).to match(
       %r{<img alt="GitLab" src="/images/mailers/gitlab_logo\.(?:gif|png)" width="\d+" height="\d+" />}
)

after

expect(header_logo).to match(default_header_logo)
# upstream will return %r{<img alt="GitLab" src="/images/mailers/gitlab_logo\.(?:gif|png)" width="\d+" height="\d+" />}
# JH will return %r{<img alt="Jihu GitLab" src="/images/mailers/gitlab_logo\.(?:gif|png)" width="\d+" height="\d+" />}