test_signup: Fix LDAP end-to-end tests accidentally using wrong backend.
test_signup: Fix LDAP end-to-end tests accidentally using wrong backend.

Apparently, we were accidentally using the EmailAuthBackend in parts
of these tests.

synicalsyntax/zulip,kou/zulip,synicalsyntax/zulip,zulip/zulip,shubhamdhama/zulip,timabbott/zulip,hackerkid/zulip,mahim97/zulip,tommyip/zulip,tommyip/zulip,hackerkid/zulip,eeshangarg/zulip,tommyip/zulip,zulip/zulip,brainwane/zulip,rishig/zulip,jackrzhang/zulip,dhcrzf/zulip,timabbott/zulip,andersk/zulip,brainwane/zulip,synicalsyntax/zulip,andersk/zulip,Galexrt/zulip,punchagan/zulip,dhcrzf/zulip,kou/zulip,eeshangarg/zulip,dhcrzf/zulip,rishig/zulip,Galexrt/zulip,hackerkid/zulip,hackerkid/zulip,dhcrzf/zulip,kou/zulip,andersk/zulip,zulip/zulip,andersk/zulip,brainwane/zulip,kou/zulip,mahim97/zulip,timabbott/zulip,tommyip/zulip,dhcrzf/zulip,showell/zulip,Galexrt/zulip,punchagan/zulip,punchagan/zulip,tommyip/zulip,eeshangarg/zulip,hackerkid/zulip,synicalsyntax/zulip,punchagan/zulip,tommyip/zulip,brainwane/zulip,eeshangarg/zulip,tommyip/zulip,rht/zulip,mahim97/zulip,jackrzhang/zulip,eeshangarg/zulip,zulip/zulip,showell/zulip,jackrzhang/zulip,punchagan/zulip,hackerkid/zulip,showell/zulip,rishig/zulip,rht/zulip,shubhamdhama/zulip,jackrzhang/zulip,Galexrt/zulip,rht/zulip,kou/zulip,mahim97/zulip,punchagan/zulip,synicalsyntax/zulip,dhcrzf/zulip,brockwhittaker/zulip,shubhamdhama/zulip,rishig/zulip,hackerkid/zulip,brainwane/zulip,rht/zulip,showell/zulip,synicalsyntax/zulip,brockwhittaker/zulip,brockwhittaker/zulip,zulip/zulip,brockwhittaker/zulip,timabbott/zulip,Galexrt/zulip,zulip/zulip,kou/zulip,shubhamdhama/zulip,jackrzhang/zulip,mahim97/zulip,synicalsyntax/zulip,rishig/zulip,Galexrt/zulip,eeshangarg/zulip,andersk/zulip,rishig/zulip,rht/zulip,mahim97/zulip,showell/zulip,dhcrzf/zulip,Galexrt/zulip,showell/zulip,shubhamdhama/zulip,timabbott/zulip,rht/zulip,showell/zulip,shubhamdhama/zulip,zulip/zulip,jackrzhang/zulip,jackrzhang/zulip,andersk/zulip,brainwane/zulip,brockwhittaker/zulip,rishig/zulip,eeshangarg/zulip,punchagan/zulip,timabbott/zulip,brainwane/zulip,brockwhittaker/zulip,timabbott/zulip,shubhamdhama/zulip,rht/zulip,andersk/zulip,kou/zulip