Force C locale in tst_Moc::notifyError()

Otherwise, the test can fail because it detects "erreur" in French
instead of the expected "error" in English.

Change-Id: I7fce02cc9a6efd2c2f4bbbf72dc328b92a1eae74
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

tests/auto/tools/moc/tst_moc.cpp

@@ -2094,6 +2094,7 @@ void tst_Moc::notifyError()
     QByteArray mocOut = proc.readAllStandardOutput();
     QVERIFY(!mocOut.isEmpty());
 
+    proc.setEnvironment({"LC_ALL=C.UTF-8"});
     QStringList args;
     args << "-c" << "-x" << "c++" << "-I" << "."
          << "-I" << qtIncludePath << "-o" << "/dev/null" << "-fPIC" << "-std=c++1z" << "-";