firestore: small test fix
firestore: small test fix

Add accidentally-unused opts to Set arg.

Change-Id: I64bc041e9989d4f65fdcd1ecf0759e508fef5dee
Reviewed-on: https://code-review.googlesource.com/c/34590
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go