QLocale and QString tests had copies of a TransientLocale; we've
recently improved the QLocale one. Rather than duplicating those
rather complicated improvements, finally share a common version.
In the process, I noticed that setlocale() only returns the prior
value when passed nullptr as the new value; so rework the
implementation to get that right, so that it now correctly restores
the prior locale. That, in turn, means there's now a later call to
setlocale(), when we actually set the changed setting, which may
invalidate the earlier return; so copy it to a QByteArray before the
second call.
Included Ivan Solovev's improved version of how to reset the locale,
since TransientLocale needs it.
Pick-to: 6.2
Change-Id: I4cb1efbda42f0e2cdd934e04b3b3732ce0f45a06
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
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