flaky test on DDLNotifier

[lance6716]

https://do.pingcap.net/jenkins/blue/organizations/jenkins/pingcap%2Ftidb%2Fghpr_check2/detail/ghpr_check2/27040/pipeline

panic: assert failed, error processing events: session pool closed [recovered]

	panic: assert failed, error processing events: session pool closed



goroutine 132297 [running]:

github.com/pingcap/tidb/pkg/ddl/notifier.(*DDLNotifier).OnBecomeOwner.func1({0x8beb040, 0xc048a9e3f0})

	pkg/ddl/notifier/subscribe.go:338 +0x45

github.com/pingcap/tidb/pkg/util.(*WaitGroupWrapper).RunWithRecover.func1.1()

	pkg/util/wait_group_wrapper.go:195 +0x43

panic({0x8beb040?, 0xc048a9e3f0?})

	GOROOT/src/runtime/panic.go:785 +0x132

github.com/pingcap/tidb/pkg/util/intest.doPanic(...)

	pkg/util/intest/assert.go:62

github.com/pingcap/tidb/pkg/util/intest.Assert(...)

	pkg/util/intest/assert.go:30

github.com/pingcap/tidb/pkg/ddl/notifier.(*DDLNotifier).start(0xc02fd50120)

	pkg/ddl/notifier/subscribe.go:164 +0x765

github.com/pingcap/tidb/pkg/util.(*WaitGroupWrapper).RunWithRecover.func1()

	pkg/util/wait_group_wrapper.go:199 +0xa6

created by github.com/pingcap/tidb/pkg/util.(*WaitGroupWrapper).RunWithRecover in goroutine 126710

	pkg/util/wait_group_wrapper.go:191 +0x125