test: allow starting/stopping txs from either main/bg threads #6147

Job	Run time
Build test server	1m 2s
Unit iOS - Thread Sanitizer	2m 52s
UI Tests for ios_swift on Simulators	9m 6s
UI Tests for ios_objc on Simulators	5m 25s
UI Tests for tvos_swift on Simulators	4m 8s
UI Tests with Address Sanitizer	9m 30s
UI Tests for SwiftUI on iPhone 8 (15.2) Simulator	6m 6s
UI Tests for SwiftUI on iPhone 8 (16.1) Simulator	9m 51s
Unit iOS - Xcode 13.2.1 - OS 13.7	4m 57s
Unit iOS - Xcode 13.2.1 - OS 14.5	5m 11s
Unit iOS - Xcode 13.4.1 - OS latest	6m 38s
Unit iOS - Xcode 14.3 - OS latest	6m 0s
Unit macOS - Xcode 13.2.1 - OS latest	3m 45s
Unit macOS - Xcode 13.4.1 - OS latest	4m 12s
Unit macOS - Xcode 14.3 - OS latest	20m 16s
Unit Catalyst - Xcode 14.3 - OS latest	4m 17s
Unit tvOS - Xcode 13.4.1 - OS latest	5m 10s
Unit tvOS - Xcode 14.3 - OS latest	4m 17s
	1h 52m 43s

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