POTENTIAL DEADLOCK in the TestTiFlashMassiveReplicaAvailable

## Bug Report

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### 1. Minimal reproduce step (Required)

### 2. What did you expect to see? (Required)

### 3. What did you see instead (Required)

```
POTENTIAL DEADLOCK:
Previous place where the lock was grabbed
goroutine 148 lock 0xc000a460e0
pkg/ddl/tests/tiflash/ddl_tiflash_test.go:109 tiflash_test.createTiFlashContext.func2 ??? <<<<<
pkg/ddl/tests/tiflash/ddl_tiflash_test.go:108 tiflash_test.createTiFlashContext.func2 ???
pkg/ddl/tests/tiflash/ddl_tiflash_test.go:552 tiflash_test.TestTiFlashMassiveReplicaAvailable ???
Have been trying to lock it again for more than 20s
goroutine 10214 lock 0xc000a460e0
pkg/domain/infosync/tiflash_manager.go:441 infosync.(*MockTiFlash).setUpMockTiFlashHTTPServer.func1 ??? <<<<<
pkg/domain/infosync/tiflash_manager.go:440 infosync.(*MockTiFlash).setUpMockTiFlashHTTPServer.func1 ???
GOROOT/src/net/http/server.go:2322 http.HandlerFunc.ServeHTTP ???
external/com_github_gorilla_mux/mux.go:212 mux.(*Router).ServeHTTP ???
GOROOT/src/net/http/server.go:3340 http.serverHandler.ServeHTTP ???
GOROOT/src/net/http/server.go:2109 http.(*conn).serve ???
Here is what goroutine 148 doing now
goroutine 148 [sync.WaitGroup.Wait]:
sync.runtime_SemacquireWaitGroup(0x0?, 0x0?)
GOROOT/src/runtime/sema.go:114 +0x2e
sync.(*WaitGroup).Wait(0xc000a46190)
GOROOT/src/sync/waitgroup.go:206 +0x85
net/http/httptest.(*Server).Close(0xc000a46150)
GOROOT/src/net/http/httptest/server.go:251 +0x245
pkg/ddl/tests/tiflash/tiflash_test.createTiFlashContext.func2()
pkg/ddl/tests/tiflash/ddl_tiflash_test.go:110 +0x5e
pkg/ddl/tests/tiflash/tiflash_test.TestTiFlashMassiveReplicaAvailable(0xc000511c00)
pkg/ddl/tests/tiflash/ddl_tiflash_test.go:553 +0x27c
testing.tRunner(0xc000511c00, 0x33d2d28)
GOROOT/src/testing/testing.go:1934 +0xea
created by testing.(*T).Run in goroutine 1
GOROOT/src/testing/testing.go:1997 +0x465

```

https://tiprow.hawkingrei.com/view/gs/pingcapprow/pr-logs/pull/pingcap_tidb/66937/fast_test_tiprow/2031951149732466688#1:build-log.txt%3A7614
### 4. What is your TiDB version? (Required)