fix(inputs.socket_listener): Fix race in tests

[srebhan]

• Updated associated README.md.
• Wrote appropriate unit tests.
• Pull request title or commits are in conventional commit format

This PR tries to fix a race in the unit-test observed on CI.

=== Failed
=== FAIL: plugins/inputs/socket_listener TestSocketListener/TCP_with_TLS (0.28s)
2023/05/19 19:23:12 I! [] Listening on tcp://127.0.0.1:49719
==================
WARNING: DATA RACE
Read at 0x00c0005a6960 by goroutine 51:
  reflect.maplen()
      C:/Program Files/Go/src/runtime/map.go:1394 +0x0
  reflect.Value.lenNonSlice()
      C:/Program Files/Go/src/reflect/value.go:1704 +0x26d
  reflect.Value.Len()
      C:/Program Files/Go/src/reflect/value.go:1693 +0x290
  github.com/stretchr/testify/assert.isEmpty()
      C:/Users/circleci.PACKER-64370BA5/go/pkg/mod/github.com/stretchr/testify@v1.8.2/assert/assertions.go:567 +0x281
  github.com/stretchr/testify/assert.NotEmpty()
      C:/Users/circleci.PACKER-64370BA5/go/pkg/mod/github.com/stretchr/testify@v1.8.2/assert/assertions.go:607 +0x5c
  github.com/stretchr/testify/require.NotEmpty()
      C:/Users/circleci.PACKER-64370BA5/go/pkg/mod/github.com/stretchr/testify@v1.8.2/require/require.go:1349 +0xa4
  github.com/influxdata/telegraf/plugins/inputs/socket_listener.TestSocketListener.func1()
      C:/Users/circleci.PACKER-64370BA5/project/plugins/inputs/socket_listener/socket_listener_test.go:195 +0x15fe
  testing.tRunner()
      C:/Program Files/Go/src/testing/testing.go:1576 +0x216
  testing.(*T).Run.func1()
      C:/Program Files/Go/src/testing/testing.go:1629 +0x47

Previous write at 0x00c0005a6960 by goroutine 54:
  runtime.mapdelete()
      C:/Program Files/Go/src/runtime/map.go:695 +0x0
  github.com/influxdata/telegraf/plugins/inputs/socket_listener.(*streamListener).closeConnection()
      C:/Users/circleci.PACKER-64370BA5/project/plugins/inputs/socket_listener/stream_listener.go:141 +0x2b4
  github.com/influxdata/telegraf/plugins/inputs/socket_listener.(*streamListener).listen.func1()
      C:/Users/circleci.PACKER-64370BA5/project/plugins/inputs/socket_listener/stream_listener.go:200 +0x204
  github.com/influxdata/telegraf/plugins/inputs/socket_listener.(*streamListener).listen.func3()
      C:/Users/circleci.PACKER-64370BA5/project/plugins/inputs/socket_listener/stream_listener.go:202 +0x58

Goroutine 51 (running) created at:
  testing.(*T).Run()
      C:/Program Files/Go/src/testing/testing.go:1629 +0x805
  github.com/influxdata/telegraf/plugins/inputs/socket_listener.TestSocketListener()
      C:/Users/circleci.PACKER-64370BA5/project/plugins/inputs/socket_listener/socket_listener_test.go:116 +0x944
  testing.tRunner()
      C:/Program Files/Go/src/testing/testing.go:1576 +0x216
  testing.(*T).Run.func1()
      C:/Program Files/Go/src/testing/testing.go:1629 +0x47

Goroutine 54 (finished) created at:
  github.com/influxdata/telegraf/plugins/inputs/socket_listener.(*streamListener).listen()
      C:/Users/circleci.PACKER-64370BA5/project/plugins/inputs/socket_listener/stream_listener.go:192 +0x4e9
  github.com/influxdata/telegraf/plugins/inputs/socket_listener.(*SocketListener).Start.func1()
      C:/Users/circleci.PACKER-64370BA5/project/plugins/inputs/socket_listener/socket_listener.go:241 +0xd8
==================
    testing.go:1446: race detected during execution of test

=== FAIL: plugins/inputs/socket_listener TestSocketListener (0.75s)
    testing.go:1446: race detected during execution of test

[telegraf-tiger]

Download PR build artifacts for linux_amd64.tar.gz, darwin_amd64.tar.gz, and windows_amd64.zip.
Downloads for additional architectures and packages are available below.

☺️ This pull request doesn't significantly change the Telegraf binary size (less than 1%)

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Artifact URLs

DEB	RPM	TAR GZ	ZIP
amd64.deb	aarch64.rpm	darwin_amd64.tar.gz	windows_amd64.zip
arm64.deb	armel.rpm	darwin_arm64.tar.gz	windows_arm64.zip
armel.deb	armv6hl.rpm	freebsd_amd64.tar.gz	windows_i386.zip
armhf.deb	i386.rpm	freebsd_armv7.tar.gz
i386.deb	ppc64le.rpm	freebsd_i386.tar.gz
mips.deb	riscv64.rpm	linux_amd64.tar.gz
mipsel.deb	s390x.rpm	linux_arm64.tar.gz
ppc64el.deb	x86_64.rpm	linux_armel.tar.gz
riscv64.deb		linux_armhf.tar.gz
s390x.deb		linux_i386.tar.gz
		linux_mips.tar.gz
		linux_mipsel.tar.gz
		linux_ppc64le.tar.gz
		linux_riscv64.tar.gz
		linux_s390x.tar.gz

[powersj]

Thanks - hopefully this gets it!