Flaky test fix: Widen timestamp tolerance in declarative tests

The 'Node timestamps' test in AbsoluteNodeTest.qml, and a similar check
in BatchReadWriteTest.qml, verified that the gap between the server
generated timestamp and the client-side 'now' was less than 10 seconds.

Widen the tolerance to 30 seconds to reduce this flakiness. This value
was validated empirically across several Coin CI integrations
covering ~30 platform configurations (Linux, Windows, macOS, QEMU-
emulated) to measure the real-world gap. Across 3 runs (99 samples),
observed gaps ranged from ~700ms up to an outlier of 16.4s on one
Windows/mingw configuration, with most configurations well under 2
seconds. 30 seconds provides a safe margin above the observed values.

Fixes: QTBUG-151059
Change-Id: I69727bff5372693d616cea16355e786cd8ec0468
Reviewed-by: Jannis Völker <jannis.voelker@basyskom.com>