TestNetworkError sometimes fails

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

make test on 23.2.0 Darwin Kernel Version 23.2.0: Wed Nov 15 21:54:55 PST 2023; root:xnu-10002.61.3~2/RELEASE_ARM64_T8122 arm64

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

All tests pass

3. What did you see instead (Required)

Sometimes TestNetworkError fails:

--- FAIL: TestNetworkError (0.01s)
    common_test.go:113: client quit, error: <nil>
    common_test.go:134: proxy quit, error: this is an error from the client connection: failed to read the connection: EOF
        github.com/pingcap/tiproxy/pkg/proxy/net.(*basicReadWriter).Read
        	/Users/zhangming/gopath/src/github.com/pingcap/tiproxy/pkg/proxy/net/packetio.go
        github.com/pingcap/tiproxy/pkg/proxy/net.ReadFull
        	/Users/zhangming/gopath/src/github.com/pingcap/tiproxy/pkg/proxy/net/packetio.go
        github.com/pingcap/tiproxy/pkg/proxy/net.(*PacketIO).readOnePacket
        	/Users/zhangming/gopath/src/github.com/pingcap/tiproxy/pkg/proxy/net/packetio.go
        github.com/pingcap/tiproxy/pkg/proxy/net.(*PacketIO).ReadPacket
        	/Users/zhangming/gopath/src/github.com/pingcap/tiproxy/pkg/proxy/net/packetio.go
        github.com/pingcap/tiproxy/pkg/proxy/net.(*PacketIO).ReadSSLRequestOrHandshakeResp
        	/Users/zhangming/gopath/src/github.com/pingcap/tiproxy/pkg/proxy/net/packetio_mysql.go
        github.com/pingcap/tiproxy/pkg/proxy/backend.(*Authenticator).handshakeFirstTime
        	/Users/zhangming/gopath/src/github.com/pingcap/tiproxy/pkg/proxy/backend/authenticator.go
        github.com/pingcap/tiproxy/pkg/proxy/backend.(*mockProxy).authenticateFirstTime
        	/Users/zhangming/gopath/src/github.com/pingcap/tiproxy/pkg/proxy/backend/mock_proxy_test.go
        github.com/pingcap/tiproxy/pkg/proxy/backend.(*tcpConnSuite).run.func3
        	/Users/zhangming/gopath/src/github.com/pingcap/tiproxy/pkg/proxy/backend/common_test.go
        github.com/pingcap/tiproxy/lib/util/waitgroup.(*WaitGroup).Run.func1
        	/Users/zhangming/gopath/src/github.com/pingcap/tiproxy/lib/util/waitgroup/waitgroup.go
        runtime.goexit
        	/opt/homebrew/Cellar/go/1.22.2/libexec/src/runtime/asm_arm64.s
    common_test.go:122: backend quit, error: <nil>
    cmd_processor_test.go:1036:
        	Error Trace:	/Users/zhangming/gopath/src/github.com/pingcap/tiproxy/pkg/proxy/backend/cmd_processor_test.go:1036
        	            				/Users/zhangming/gopath/src/github.com/pingcap/tiproxy/pkg/proxy/backend/testsuite_test.go:175
        	            				/Users/zhangming/gopath/src/github.com/pingcap/tiproxy/pkg/proxy/backend/testsuite_test.go:181
        	            				/Users/zhangming/gopath/src/github.com/pingcap/tiproxy/pkg/proxy/backend/cmd_processor_test.go:1043
        	Error:      	Should be true
        	Test:       	TestNetworkError
FAIL

4. What is your version? (Required)

master

Run make test and focus on TestNetworkError in pkg/proxy/backend/common_test.go. Read the failure path through cmd_processor_test.go, testsuite_test.go, and the referenced packet I/O files. Done means the intermittent test failure is understood and TestNetworkError passes reliably.