roachtest: deflake cdc/ledger

This patch deflakes `cdc/ledger` by increasing the target steady latency
from 1 minute to 3 minutes. It also eliminates a weird quirk where the
latency verifier expects the latency to be less than half of the target
latency before it'll be considered steady, which led to weird logs and
added unnecessary complexity.

Release note: None

Author: andyyang890

pkg/cmd/roachtest/tests/cdc.go

@@ -2574,7 +2574,7 @@ CONFIGURE ZONE USING
 			})
 			ct.runFeedLatencyVerifier(feed, latencyTargets{
 				initialScanLatency: 10 * time.Minute,
-				steadyLatency:      time.Minute,
+				steadyLatency:      3 * time.Minute,
 			})
 			ct.waitForWorkload()
 		},

pkg/cmd/roachtest/tests/latency_verifier.go

@@ -116,7 +116,7 @@ func (lv *latencyVerifier) noteHighwater(highwaterTime time.Time) {
 		return
 	}
 
-	if lv.targetSteadyLatency == 0 || latency < lv.targetSteadyLatency/2 {
+	if lv.targetSteadyLatency == 0 || latency < lv.targetSteadyLatency {
 		lv.latencyBecameSteady = true
 	}
 	if !lv.latencyBecameSteady {