Change the time-getting logic in xds test to what ExecCtx does (#27467)

test/cpp/end2end/xds_end2end_test.cc

@@ -1648,8 +1648,7 @@ class NoOpHttpFilter : public grpc_core::XdsHttpFilterImpl {
 // GPR round the number at millisecond-level. This creates a 1ms difference,
 // which could cause flake.
 grpc_millis NowFromCycleCounter() {
-  gpr_cycle_counter now = gpr_get_cycle_counter();
-  return grpc_cycle_counter_to_millis_round_up(now);
+  return grpc_timespec_to_millis_round_down(gpr_now(GPR_CLOCK_MONOTONIC));
 }
 
 // Returns the number of RPCs needed to pass error_tolerance at 99.99994%