imports: adjust vendored packages test to keep working against 1.6

The prior commit fixed this test for tip (1.7) but broke it for 1.6.

Change-Id: Ic39d215ac8bc60dba4b66201f1f16713b2e02d9c
Reviewed-on: https://go-review.googlesource.com/21144
Reviewed-by: Alan Donovan <adonovan@google.com>
1 file changed