spanconfigtestutils: copy gc job testing knobs to tenant

Some of the sqltranslator tests require that we can disable the GC job
related to an index creation so that we can observe the span
configuration on the temporary indexes.  We pause the GC job via a
testing knob.

Release justification: Test-only change

Release note: None

pkg/spanconfig/spanconfigtestutils/spanconfigtestcluster/cluster.go

@@ -63,13 +63,17 @@ func (h *Handle) InitializeTenant(ctx context.Context, tenID roachpb.TenantID) *
 		tenantState.db = sqlutils.MakeSQLRunner(h.tc.ServerConn(0))
 		tenantState.cleanup = func() {} // noop
 	} else {
+		serverGCJobKnobs := testServer.TestingKnobs().GCJob
+		tenantGCJobKnobs := sql.GCJobTestingKnobs{SkipWaitingForMVCCGC: true}
+		if serverGCJobKnobs != nil {
+			tenantGCJobKnobs = *serverGCJobKnobs.(*sql.GCJobTestingKnobs)
+			tenantGCJobKnobs.SkipWaitingForMVCCGC = true
+		}
 		tenantArgs := base.TestTenantArgs{
 			TenantID: tenID,
 			TestingKnobs: base.TestingKnobs{
 				SpanConfig: h.scKnobs,
-				GCJob: &sql.GCJobTestingKnobs{
-					SkipWaitingForMVCCGC: true,
-				},
+				GCJob:      &tenantGCJobKnobs,
 			},
 		}
 		var err error