kvnemesis: fix and unskip TestKVNemesisMultiNode

Before this patch, cockroachdb#56197 broke this test because it changed a range
merge error string that let the KV nemesis validator ignore the error.
This patch updates the regexp the validator uses to match the error and
unskips the test.

Fixes cockroachdb#58624.

Release note: None

pkg/kv/kvnemesis/kvnemesis_test.go

@@ -55,7 +55,6 @@ func TestKVNemesisSingleNode(t *testing.T) {
 
 func TestKVNemesisMultiNode(t *testing.T) {
 	defer leaktest.AfterTest(t)()
-	skip.WithIssue(t, 58624, "flaky test")
 	skip.UnderRace(t)
 
 	defer log.Scope(t).Close(t)

pkg/kv/kvnemesis/validator.go

@@ -310,7 +310,8 @@ func (v *validator) processOp(txnID *string, op Operation) {
 			// However, I think the right thing to do is sniff this inside the
 			// AdminMerge code and retry so the client never sees it. In the meantime,
 			// no-op. #44377
-		} else if resultIsError(t.Result, `merge failed: cannot merge range with non-voter replicas`) {
+		} else if resultIsError(t.Result,
+			`merge failed: cannot merge ranges when (rhs)|(lhs) is in a joint state or has learners`) {
 			// This operation executed concurrently with one that was changing
 			// replicas.
 		} else if resultIsError(t.Result, `merge failed: ranges not collocated`) {