Test: remove QSKIP from tst_qstring

Test: remove QSKIP from tst_qstring

Instead omit the whole tests when appropriate.

In particular:

- When Q_CC_HPACC is defined the tests fromStdString and toStdString are
crashing. Omit the tests in this configuration since the compiler is not
supported.

- Clean the localeAwareCompare() by removing the code where Q_OS_WIN is
defined but not Q_OS_WINCE. System and user locale cannot be set on
Q_OS_WIN other than Win CE and some code could never be reached.

Change-Id: I72ae3246bf8c2a73d14cce45dde14bcb8001d8b3
Reviewed-by: Sergio Ahumada <sergio.ahumada@digia.com>