Autotest: fix silly mistake in assigning where a comparison was intended

The file descriptor has been closed and this test is checking if we get
EBADF.

Change-Id: I33dc971f005a4848bb8ffffd1478eaffd99aa2e9
Reviewed-by: Jake Petroules <jake.petroules@qt.io>
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@theqtcompany.com>

tests/auto/corelib/io/qfile/tst_qfile.cpp

@@ -3446,7 +3446,7 @@ void tst_QFile::autocloseHandle()
         //file is closed, read should fail
         char buf;
         QCOMPARE((int)QT_READ(fd, &buf, 1), -1);
-        QVERIFY(errno = EBADF);
+        QVERIFY(errno == EBADF);
     }
 
     {