Update web backend to support test mode #4418

Job	Run time
Pre-commit checks / Pre-commit Checks	47s
Check towncrier / Towncrier Checks	27s
Python Package (cocoa) / Create Python Package	1m 30s
Python Package (android) / Create Python Package	53s
Python Package (core) / Create Python Package	1m 30s
Python Package (demo) / Create Python Package	1m 5s
Python Package (dummy) / Create Python Package	57s
Python Package (gtk) / Create Python Package	51s
Python Package (iOS) / Create Python Package	1m 52s
Python Package (textual) / Create Python Package	1m 39s
Python Package (toga) / Create Python Package	52s
Python Package (web) / Create Python Package	1m 15s
Python Package (winforms) / Create Python Package	1m 6s
core	0s
Combine & check core coverage.	0s
testbed	0s
	14m 44s

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