Fix failing tst_qqmlecmascript::qtbug_22843:"with .pragma library"

Try to to load a .pragma library type of script only once, even if
it has a syntax error.

Change-Id: I32d7fb71647f413dc75e339e7bff8d74f7754c87
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
Fix tst_qjsengine::jsIncDecNonObjectProperty

With pre increment/decrement expressions, don't rely on the
target expr to be readable after the write to it. In case of
++someString.length for example the value of the property may not
change, but the result of the expression still has to be the
incremented value.

Change-Id: I2389eeee38dbed75aa2764638cb40950e32b8a68
Reviewed-by: Lars Knoll <lars.knoll@digia.com>