Add prepend mod to terms_helper.rb

What does this MR do and why?

Some test cases of spec/features/users/login_spec.rb failed on JH side

Failure/Error: expect(page).to have_content('Please accept the Terms of Service before continuing.')
       expected to find text "Please accept the Terms of Service before continuing." in "Please verify your phone before continuing. These are the terms If you do not agree with these conditions, you can contact Jihu GitLab support team for helping you with your current account Phone +86 +852 +853 +886 +1 +81 +60 +61 +44 +65 +49 +7 +20 +27 +30 +31 +32 +33 +34 +36 +39 +40 +41 +43 +45 +46 +47 +48 +52 +53 +54 +55 +62 +63 +64 +66 +82 +84 +90 +91 +92 +95 +351 +352 +353 +354 +355 +356 +357 +358 +359 +370 +371 +372 +373 +374 +375 +376 +377 +378 +380 +386 +420 +421 +423 +855 +856 +880 +966 +967 +968 +971 +972 +973 +974 +976 +381 Verification code Get code Decline and sign out Accept terms"
     # ./spec/support/helpers/terms_helper.rb:19:in `expect_to_be_on_terms_page'
     # ./spec/features/users/login_spec.rb:817:in `block (5 levels) in <top (required)>'
     # ./spec/spec_helper.rb:423:in `block (3 levels) in <top (required)>'
     # ./spec/support/sidekiq_middleware.rb:9:in `with_sidekiq_server_middleware'
     # ./spec/spec_helper.rb:414:in `block (2 levels) in <top (required)>'
     # ./spec/spec_helper.rb:410:in `block (3 levels) in <top (required)>'
     # ./lib/gitlab/application_context.rb:45:in `with_raw_context'
     # ./spec/spec_helper.rb:410:in `block (2 levels) in <top (required)>'
     # ./spec/spec_helper.rb:267:in `block (2 levels) in <top (required)>'
     # ./spec/support/system_exit_detected.rb:7:in `block (2 levels) in <top (required)>'
     # ./spec/support/redis.rb:53:in `block (2 levels) in <top (required)>'
     # ./spec/support/flaky_tests.rb:27:in `block (2 levels) in <top (required)>'
     # ./spec/support/database/prevent_cross_joins.rb:106:in `block (3 levels) in <top (required)>'
     # ./spec/support/database/prevent_cross_joins.rb:60:in `with_cross_joins_prevented'
     # ./spec/support/database/prevent_cross_joins.rb:106:in `block (2 levels) in <top (required)>'
     # ./jh/spec/spec_helper.rb:15:in `block (2 levels) in <top (required)>'

So we wanna override the method expect_to_be_on_terms_page of ./spec/support/helpers/terms_helper.rb and we add prepend_mode to this file.

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