ExchangeContext leak in Darwin unit test

[kghost]

Problem

https://github.com/project-chip/connectedhomeip/pull/5389/checks?check_run_id=2554419054

2021-05-11 10:19:12.280249+0000 xctest[9231:42649] Thermostat SystemMode Error: (null)
Test Case '-[CHIPClustersTests testSendClusterThermostatWriteAttributeSystemModeWithValue]' passed (0.004 seconds).
Test Case '-[CHIPClustersTests testShutdownStack]' started.
2021-05-11 10:19:12.281295+0000 xctest[9231:42649] PersistentStorageDelegate Delete Key: PairedDevice1
2021-05-11 10:19:12.281862+0000 xctest[9231:42649] PersistentStorageDelegate Set Key ListPairedDevices0
2021-05-11 10:19:12.282071+0000 xctest[9231:42649] Shutting down the CHIP Stack
[1620728352282] [0xa699] CHIP: [CTL] Shutting down the commissioner
[1620728352282] [0xa699] CHIP: [CTL] Shutting down the controller
Assertion failed: (ec.GetReferenceCount() == 0), function Shutdown, file ../../../../../../../../../../../work/connectedhomeip/connectedhomeip/src/messaging/ExchangeMgr.cpp, line 104.
2021-05-11 10:19:33.043 xcodebuild[8074:43675]  IDETestOperationsObserverDebug: Writing diagnostic log for test session to:
/Users/runner/Library/Developer/Xcode/DerivedData/CHIP-dhyjrdlylemwdfesoclvgithlmec/Logs/Test/Test-CHIP Framework Tests-2021.05.11_10-17-34-+0000.xcresult/Staging/1_Test/Diagnostics/CHIPTests-DBE91E36-1FF3-4813-9B0B-6A4014870F80/CHIPTests-3324E36C-FE7B-4E61-A3B2-2681A842D48A/Session-CHIPTests-2021-05-11_101933-rd5sts.log
2021-05-11 10:19:33.043 xcodebuild[8074:38898] [MT] IDETestOperationsObserverDebug: (36B55EE0-E532-4205-BEAF-BF1EA7CE8F6E) Beginning test session CHIPTests-36B55EE0-E532-4205-BEAF-BF1EA7CE8F6E at 2021-05-11 10:19:33.043 with Xcode 12D4e on target <DVTLocalComputer: 0x7fe6e9ca61c0 (My Mac | x86_64)> (10.15.7 (19H1030))

There is an assert fail while shutting down the exchange manager.