[devtools] Make timeout error in qunit tests configurable and use SkipTest in test_jscript.py
This is grafted from 87443f279b0f in default branch (got tired of CI failing
because of these unreliable tests).
[web] Handle unspecified value in TZDatetimeField
`field._ensure_correctly_typed(self, form, value)` should return None if value
is None.
Closes #17050181.