kv: un-skip most of TestMergeQueue

Informs cockroachdb#63009.
Informs cockroachdb#64056.

In cockroachdb#64199, we found that the flake was likely due to the non-collocated
subtest, so this commit un-skips the parent test and skips only this
single subtest.

pkg/kv/kvserver/client_merge_test.go

@@ -4224,6 +4224,7 @@ func TestMergeQueue(t *testing.T) {
 	})
 
 	t.Run("non-collocated", func(t *testing.T) {
+		skip.WithIssue(t, 64056, "flaky test")
 		reset(t)
 		verifyUnmerged(t, store, lhsStartKey, rhsStartKey)
 		tc.AddVotersOrFatal(t, rhs().Desc().StartKey.AsRawKey(), tc.Target(1))