qmimetype: Fix test failure when LC_ALL is set.

This code sets LANG=en_US so that the method comment(), which returns a translated
name, can be compared with an expected result in English.
(QMimeType::comment uses QLocale::system().name() and QLocale::system().uiLanguages())

But LANG= has no effect if LC_ALL is set, so LC_ALL needs to be cleared (or set
to en_US) for the test to work.

Change-Id: Icb031057769be9bc8c0fcab65daa45e7bf1d5b18
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

tests/auto/corelib/mimetypes/qmimedatabase/tst_qmimedatabase.cpp

@@ -51,6 +51,7 @@
 
 void initializeLang()
 {
+    qputenv("LC_ALL", "");
     qputenv("LANG", "en_US");
 }