overlay: add unionmount tests with multi lower layers

The tests with multi lower layers rotate the upper layer into a lower
layer on specific operations such as mkdir, rename and link.
That provides test coverage for redirect_dir and index features.

The following variations are covered:
1. All layers all on scratch fs (--ov=10 --samefs)
2. All layers on scratch fs expect for lowermost on test fs (--ov=10)
3. Lowermost layer on test fs, one layer on tmpfs and the rest
   on sratch fs (--ov=10 --maxfs=1)
4. Lowermost layer on test fs, one layer on scratch fs and the rest
   are unique tmpfs instances (--ov=10 --maxfs=10)

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>

tests/overlay/107

@@ -0,0 +1,41 @@
+#! /bin/bash
+# SPDX-License-Identifier: GPL-2.0
+# Copyright (C) 2020 CTERA Networks. All Rights Reserved.
+#
+# FS QA Test 107
+#
+# Run unionmount testsuite to verify correctness
+# with multi lower layers, some layers on unique fs,
+# one layer is on tmpfs with xino enabled.
+#
+seq=`basename $0`
+seqres=$RESULT_DIR/$seq
+echo "QA output created by $seq"
+
+here=`pwd`
+tmp=/tmp/$$
+status=1	# failure is the default!
+trap "_unionmount_testsuite_cleanup; exit \$status" 0 1 2 3 15
+
+# get standard environment, filters and checks
+. ./common/rc
+. ./common/filter
+
+# remove previous $seqres.full before test
+rm -f $seqres.full
+
+# real QA test starts here
+
+_supported_fs overlay
+_supported_os Linux
+_require_tmpfs
+_require_test
+_require_scratch
+_require_unionmount_testsuite
+
+_unionmount_testsuite_run --ov=10 --maxfs=1 --xino --verify
+
+# success, all done
+echo "Silence is golden"
+status=0
+exit