test: fix tst_QLocale::macDefaultLocale

test: fix tst_QLocale::macDefaultLocale

1. There is a behaviour change for CFDateFormatterGetFormat() between
10.6 and later, QLocale::dateFormat(QLocale::LongFormat) will return
"MMMM d, yyyy" for 10.6 and "MMMM d, y" for 10.7, 10.8
2. Add a comment for toCurrencyString() test, need another system
settings

Task-number: QTBUG-27790
Change-Id: I4fe684d6e0c1d4a140e3b1f1ef395b7fdad030b4
Reviewed-by: Samuli Piippo <samuli.piippo@digia.com>
Reviewed-by: Janne Anttila <janne.anttila@digia.com>
Reviewed-by: Sergio Ahumada <sergio.ahumada@digia.com>