roachtest: fix follower-reads/survival=region

Previously, this test allocated 6 nodes: 3 in us-east1, 2 in us-west1,
and 1 in europe-west2. When using survival regions, we would have 5
replicas and 2 would be allowed to be virtually anywhere without
constraints. However, this proved problematic in serverless
environments, as the additional regions could fall outside the user's
configured regions. We later modified this logic to use the same
constraints as super regions, which now forces one region to have more
replicas. This patch adjusts the test so that the Europe region has one
more replica to account for this logic.

Fixes: #143333
Fixes: #143199

Release note: None

Author: fqazi

pkg/cmd/roachtest/tests/follower_reads.go

@@ -56,7 +56,7 @@ func registerFollowerReads(r registry.Registry) {
 				6, /* nodeCount */
 				spec.CPU(4),
 				spec.Geo(),
-				spec.GCEZones("us-east1-b,us-east1-b,us-east1-b,us-west1-b,us-west1-b,europe-west2-b"),
+				spec.GCEZones("us-east1-b,us-east1-b,us-east1-b,us-west1-b,europe-west2-b,europe-west2-b"),
 			),
 			CompatibleClouds: registry.OnlyGCE,
 			Suites:           registry.Suites(registry.Nightly),