Unblacklist changeLocale auto tests

Do the error message comparison with utf-8 enconding since the localized
error messages are stored in utf-8 format (error_page_strings.grdp).
This also stabilizes the test because it doesn't depend on the encoding
used by the Chromium.

Change-Id: I24addc1c18d5b4325f4c0270e675e02c3f4e0812
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>