tests: fix mock server sends event before kv client is ready

[amyangfei]

What problem does this PR solve?

Fix unstable unit test

[2021-04-21T06:27:22.638Z] [2021/04/21 14:26:58.418 +08:00] [ERROR] [client_v2.go:81] ["received an event but neither pending region nor running region was found"] [regionID=3] [requestID=61] [addr=127.0.0.1:15936] [stack="github.com/pingcap/ticdc/cdc/kv.(*eventFeedSession).sendRegionChangeEventV2\n\t/home/jenkins/agent/workspace/cdc_ghpr_leak_test/go/src/github.com/pingcap/ticdc/cdc/kv/client_v2.go:81\ngithub.jparrowsec.cn/pingcap/ticdc/cdc/kv.(*eventFeedSession).receiveFromStreamV2\n\t/home/jenkins/agent/workspace/cdc_ghpr_leak_test/go/src/github.com/pingcap/ticdc/cdc/kv/client_v2.go:274\ngithub.jparrowsec.cn/pingcap/ticdc/cdc/kv.(*eventFeedSession).dispatchRequest.func1\n\t/home/jenkins/agent/workspace/cdc_ghpr_leak_test/go/src/github.com/pingcap/ticdc/cdc/kv/client.go:815\ngolang.org/x/sync/errgroup.(*Group).Go.func1\n\t/go/pkg/mod/golang.org/x/[email protected]/errgroup/errgroup.go:57"]
[2021-04-21T06:27:22.638Z] 
[2021-04-21T06:27:22.638Z] ----------------------------------------------------------------------
[2021-04-21T06:27:22.638Z] FAIL: client_test.go:1306: etcdSuite.TestStreamRecvWithErrorAndResolvedGoBack
[2021-04-21T06:27:22.639Z] 
[2021-04-21T06:27:22.639Z] client_test.go:1347:
[2021-04-21T06:27:22.639Z] client_test.go:239:
[2021-04-21T06:27:22.639Z]     s.c.Assert(err, check.IsNil)
[2021-04-21T06:27:22.639Z] ... value *status.statusError = &status.statusError{Code:1, Message:"context canceled", Details:[]*any.Any(nil), XXX_NoUnkeyedLiteral:struct {}{}, XXX_unrecognized:[]uint8(nil), XXX_sizecache:0} ("rpc error: code = Canceled desc = context canceled")
[2021-04-21T06:27:22.639Z] 
[2021-04-21T06:27:22.639Z]     c.Assert(errors.Cause(err), check.Equals, context.Canceled)
[2021-04-21T06:27:22.639Z] ... obtained *errors.Error = &errors.Error{code:0, codeText:"CDC:ErrNoPendingRegion", message:"received event regionID %v, requestID %v from %v, but neither pending region nor running region was found", redactArgsPos:[]int(nil), cause:error(nil), args:[]interface {}{0x3, 0x3d, "127.0.0.1:15936"}, file:"/home/jenkins/agent/workspace/cdc_ghpr_leak_test/go/src/github.com/pingcap/ticdc/cdc/kv/client_v2.go", line:85} ("[CDC:ErrNoPendingRegion]received event regionID 3, requestID 61 from 127.0.0.1:15936, but neither pending region nor running region was found")
[2021-04-21T06:27:22.639Z] ... expected *errors.errorString = &errors.errorString{s:"context canceled"} ("context canceled")

What is changed and how it works?

The root cause is new request ID is allocated in kv client first, and then pending region is inserted, when the test code detects request ID is ready, the pending region could be not ready.
This is a fix for the test

Check List

Tests

• Unit test
• Integration test

Release note

• No release note

[amyangfei]

/run-all-tests

[amyangfei]

ptal @liuzix @jinlingchristopher

[zier-one]

we should try to simplify testing of kvclient, but LGTM

/lgtm

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• lance6716
• leoppro

To complete the pull request process, please ask the reviewers in the list to review by filling /cc @reviewer in the comment.
After your PR has acquired the required number of LGTMs, you can assign this pull request to the committer in the list by filling /assign @committer in the comment to help you merge this pull request.

The full list of commands accepted by this bot can be found here.

Reviewer can indicate their review by writing /lgtm in a comment.
Reviewer can cancel approval by writing /lgtm cancel in a comment.

[zier-one]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: ec3d19b

[ti-srebot]

cherry pick to release-4.0 in PR #1690

[ti-srebot]

cherry pick to release-5.0 in PR #1691