UT failure in `TestHeartbeatSendFailed`

[peterbroadhurst]

https://github.com/hyperledger/firefly/runs/5096544001?check_suite_focus=true#step:4:589

time="2022-02-07T16:45:36Z" level=info msg="WS ws://127.0.0.1:36313/test/updated connected"
time="2022-02-07T16:45:37Z" level=error msg="WS ws://127.0.0.1:36313/test/updated closed: read tcp 127.0.0.1:38120->127.0.0.1:36313: use of closed network connection"
time="2022-02-07T16:45:37Z" level=warning msg="WS ws://127.0.0.1:44971 connect attempt 1 failed [500]: {\"error\": \"pop\"}"
time="2022-02-07T16:45:37Z" level=warning msg="WS ws://127.0.0.1:37681 connect attempt 1 failed [-1]: "
time="2022-02-07T16:45:37Z" level=error msg="WS  closed: read tcp 127.0.0.1:33106->127.0.0.1:45581: use of closed network connection"
time="2022-02-07T16:45:37Z" level=error msg="WS  heartbeat send failed: write tcp 127.0.0.1:33106->127.0.0.1:45581: use of closed network connection"
time="2022-02-07T16:45:37Z" level=warning msg="WS  connect attempt 1 failed [-1]: "
time="2022-02-07T16:45:37Z" level=error msg="WS  send failed: write tcp 127.0.0.1:38446->127.0.0.1:34249: use of closed network connection"
time="2022-02-07T16:45:37Z" level=error msg="WS  closing: FF10330: Websocket heartbeat timed out after 0.01ms"
time="2022-02-07T16:45:37Z" level=info msg="WS ws://127.0.0.1:3[609](https://github.com/hyperledger/firefly/runs/5096544001?check_suite_focus=true#step:4:609)9 connected"
time="2022-02-07T16:45:37Z" level=error msg="WS ws://127.0.0.1:36099 closed: read tcp 127.0.0.1:51966->127.0.0.1:36099: use of closed network connection"
time="2022-02-07T16:45:37Z" level=warning msg="WS ws://127.0.0.1:36099 connect attempt 1 failed [409]: "
time="2022-02-07T16:45:37Z" level=warning msg="WS ws://127.0.0.1:36099 connect attempt 2 failed [409]: "
time="2022-02-07T16:45:37Z" level=warning msg="WS ws://127.0.0.1:36099 connect attempt 3 failed [409]: "
time="2022-02-07T16:45:37Z" level=warning msg="WS ws://127.0.0.1:36099 connect attempt 4 failed [409]: "
time="2022-02-07T16:45:37Z" level=warning msg="WS ws://127.0.0.1:36099 connect attempt 5 failed [409]: "
time="2022-02-07T16:45:37Z" level=warning msg="WS ws://127.0.0.1:36099 connect attempt 6 failed [409]: "
time="2022-02-07T16:45:37Z" level=warning msg="WS ws://127.0.0.1:36099 connect attempt 7 failed [409]: "
time="2022-02-07T16:45:37Z" level=warning msg="WS ws://127.0.0.1:36099 connect attempt 8 failed [409]: "
time="2022-02-07T16:45:37Z" level=warning msg="WS ws://127.0.0.1:36099 connect attempt 9 failed [409]: "
time="2022-02-07T16:45:37Z" level=warning msg="WS ws://127.0.0.1:36099 connect attempt 10 failed [409]: "
time="2022-02-07T16:45:37Z" level=warning msg="WS ws://127.0.0.1:36099 connect attempt 11 failed [409]: "
time="2022-02-07T16:45:37Z" level=warning msg="WS ws://127.0.0.1:36099 connect attempt 12 failed [409]: "
time="2022-02-07T16:45:37Z" level=warning msg="WS ws://127.0.0.1:36099 connect attempt 13 failed [409]: "
time="2022-02-07T16:45:37Z" level=warning msg="WS ws://127.0.0.1:36099 connect attempt 14 failed [409]: "
time="2022-02-07T16:45:37Z" level=warning msg="WS ws://127.0.0.1:36099 connect attempt 15 failed [409]: "
--- FAIL: TestHeartbeatSendFailed (0.01s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x10 pc=0x6e4b37]

goroutine 69 [running]:
testing.tRunner.func1.2(0x7e2800, 0xae3f10)
	/opt/hostedtoolcache/go/1.16.13/x64/src/testing/testing.go:1143 +0x332
testing.tRunner.func1(0xc00041f680)
	/opt/hostedtoolcache/go/1.16.13/x64/src/testing/testing.go:1146 +0x4b6
panic(0x7e2800, 0xae3f10)
	/opt/hostedtoolcache/go/1.16.13/x64/src/runtime/panic.go:965 +0x1b9
github.com/gorilla/websocket.(*Conn).WriteMessage(0x0, 0x9, 0xb24e80, 0x0, 0x0, 0xc000343801, 0x1)
	/home/runner/go/pkg/mod/github.com/gorilla/websocket@v1.4.2/conn.go:753 +0x37
github.com/hyperledger/firefly/pkg/wsclient.(*wsClient).sendLoop(0xc0001b6b40, 0xc000438e40)
	/home/runner/work/firefly/firefly/pkg/wsclient/wsclient.go:295 +0x574
github.com/hyperledger/firefly/pkg/wsclient.TestHeartbeatSendFailed(0xc00041f680)
	/home/runner/work/firefly/firefly/pkg/wsclient/wsclient_test.go:327 +0x2cf
testing.tRunner(0xc00041f680, 0x880a60)
	/opt/hostedtoolcache/go/1.16.13/x64/src/testing/testing.go:1193 +0xe5
created by testing.(*T).Run
	/opt/hostedtoolcache/go/1.16.13/x64/src/testing/testing.go:1238 +0x2b3
FAIL	github.com/hyperledger/firefly/pkg/wsclient	0.080s
ok  	github.com/hyperledger/firefly/cmd	0.033s	coverage: 100.0% of statements
FAIL