btrfs-progs: tests: add test case to verify convert rollback output

The new new test case is to make sure the rollback output for a fixed
content converted fs contains the string "ext2_saved/image".

As we have a bug in the past where after the string "ext2_saved", we can
have some unterminated garbage.

Signed-off-by: Qu Wenruo <wqu@suse.com>

tests/convert-tests/026-rollback-output/test.sh

@@ -0,0 +1,20 @@
+#!/bin/bash
+# Make sure "btrfs-convert -r" is outputting the correct filename
+
+source "$TEST_TOP/common" || exit
+source "$TEST_TOP/common.convert" || exit
+
+setup_root_helper
+prepare_test_dev
+
+check_global_prereq mkfs.ext4
+check_prereq btrfs-convert
+check_prereq btrfs
+
+convert_test_prep_fs ext4 mke2fs -t ext4 -b 4096
+run_check_umount_test_dev
+convert_test_do_convert
+
+# Rollback and save the output.
+run_check_stdout "$TOP/btrfs-convert" --rollback "$TEST_DEV" | \
+	grep -q "ext2_saved/image" || _fail "rollback filename output is corruptedd"