[libcalamares] Fix tests

- the test checks that the default locale is C or en_US .. let's just
  make it so instead of relying on the environment. This fixes tests
  on my dev-laptop, which happens to be set to en_NL (with volapuk
  date format).

src/libcalamares/locale/Tests.cpp

@@ -37,6 +37,7 @@ void
 LocaleTests::initTestCase()
 {
     // Otherwise plain get() is dubious in the TranslatableConfiguration tests
+    QLocale::setDefault( QLocale( QStringLiteral( "en_US" ) ) );
     QVERIFY( ( QLocale().name() == "C" ) || ( QLocale().name() == "en_US" ) );
 }