Fix some QJSValue test failures.

Fix precision issues causing test failures on platforms where
qreal is defined as single precision. Also use explicit casts
to ensure well defined behaviour when converting a negative
double value to an unsigned integer.

Change-Id: Ia0048bf83169d3b617f70828f86368c23f4f3786
Reviewed-by: Chris Adams <christopher.adams@nokia.com>
Reviewed-by: Martin Jones <martin.jones@nokia.com>