internal/frontend: check HTML substrings in order
internal/frontend: check HTML substrings in order

When testing the server's output for desired substrings,
check that they appear in the order provided in the test.

Change-Id: I205b548ed048ed378fbe797b722829473506dcee
Reviewed-on: https://team-review.git.corp.google.com/c/golang/discovery/+/543858
Reviewed-by: Julie Qiu <e6cb64861edcc38971590ff27c111e0e672de045@google.com>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite