btrfs-progs: tests: Add test for collision DIR_ITEM handling

Verify that if we have an otherwise clean filesystem, containging
collided DIR_ITEM, btrfs check lowmem's mode can correctly handle those
and not produce any false positives.

This if fixed by commit titled:

 "btrfs-progs: check: fix DIR_ITEM checking in lowmem"

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

tests/fsck-tests/033-lowmem-collission-dir-items/test.sh

@@ -0,0 +1,24 @@
+#!/bin/bash
+# Ensure that running btrfs check on a fs which has name collisions of files
+# doesn't result in false positives. This test is specifically targeted at
+# lowmem mode.
+
+source "$TEST_TOP/common"
+
+check_prereq btrfs
+check_prereq mkfs.btrfs
+
+setup_root_helper
+prepare_test_dev
+
+run_check $SUDO_HELPER "$TOP/mkfs.btrfs" -f "$TEST_DEV"
+run_check_mount_test_dev
+
+# Create 2 files whose names collide
+run_check $SUDO_HELPER touch "$TEST_MNT/5ab4e206~~~~~~~~XVT1U3ZF647YS2PD4AKAG826"
+run_check $SUDO_HELPER touch "$TEST_MNT/5ab4e26a~~~~~~~~AP1C3VQBE79IJOTVOEZIR9YU"
+
+run_check_umount_test_dev
+
+# The fs is clean so lowmem shouldn't produce any warnings
+run_check "$TOP/btrfs" check --readonly "$TEST_DEV"