test: Make test_sqfs_ls() single-threaded

This test seems to interfere with the other test in this file. Mark it
single-threaded to avoid any problems.

Signed-off-by: Simon Glass <sjg@chromium.org>

test/py/tests/test_fs/test_squashfs/test_sqfs_ls.py

@@ -105,6 +105,7 @@ def sqfs_run_all_ls_tests(u_boot_console):
 @pytest.mark.buildconfigspec('cmd_squashfs')
 @pytest.mark.buildconfigspec('fs_squashfs')
 @pytest.mark.requiredtool('mksquashfs')
+@pytest.mark.singlethread
 def test_sqfs_ls(u_boot_console):
     """ Executes the sqfsls test suite.