luci-spanner: change how authentication to spanner occurs in tests.
luci-spanner: change how authentication to spanner occurs in tests.

Pass token source using WithTokenSource instead of as a GRPCDialOption
to avoid GCP attempting to find default application credentials.

BUG=None

Change-Id: Id18062710c2083d06c86dcda56c3f8c55a523174
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2992431
Reviewed-by: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>
Reviewed-by: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>
Commit-Queue: Patrick Meiring <496ae7b7898d641db15cbb7f3340e9390316ae10@google.com>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go