Fix intermittently failing test: TestOrderCreate/MultipleItemsWithDownloads

[vbrown608]

The test suite sometimes fails during CI with

    --- FAIL: TestOrderCreate/MultipleItemsWithDownloads (0.12s)
        order_test.go:299: 
            	Error Trace:	order_test.go:299
            	Error:      	"[{cdb56e41-a3fb-4821-b3a2-17305568ea0b 61c9db20-b8c8-49f8-8121-f98d88ce8463 %!s(int64=0) First Download 123-i-can-fly-456  /assets/first-download %!s(uint64=0) 0001-01-01 00:00:00 +0000 UTC 2020-03-09 12:11:08.957511864 +0000 UTC <nil>}]" should have 2 item(s), but has 1
            	Test:       	TestOrderCreate/MultipleItemsWithDownloads