billing: Add test for subscribing customer to second plan.
billing: Add test for subscribing customer to second plan.

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