Add JS test for nested postbacks #2684

Job	Run time
Build all projects without errors	11m 42s
Build published projects without warnings (Release)	4m 33s
.NET unit tests (ubuntu-latest)	4m 16s
Build published projects without warnings (Debug)	4m 32s
.NET unit tests (windows-2022)	10m 49s
.NET unit tests (macOS-latest)	6m 5s
JS unit tests	1m 47s
UI tests (chrome, ubuntu-latest, Production, Default)	14m 56s
UI tests (chrome, windows-2022, Development, Default)	27m 35s
UI tests (firefox, ubuntu-latest, Development, Default)	21m 23s
UI tests (firefox, ubuntu-latest, Production, Default)	19m 51s
UI tests (firefox, ubuntu-latest, Development, ExperimentalFeaturesTests)	22m 46s
UI tests (chrome, ubuntu-latest, Production, ExperimentalFeaturesTests)	16m 20s
	2h 46m 35s

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