sweet: don't start diagnostics until all cockroach instances are ready

There's currently a flake with TestSweetEndToEnd, specifically with
testing pgo, wherein we'll start grabbing diagnostics from cockroach
instances too soon.

Change-Id: I7eda15ee8d3401c591ad568b8c8bb125ace72e80
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/614535
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>