Add test for .filesystems

[pevik]

Filesystem setup change cce6188 would deserve a test. Some possible examples of more filesystems or single filesystem with various options.

static struct tst_test test = {
	...
	.filesystems = (struct tst_fs []) {
		{.type = "ext3"},
		{.type = "ext4"},
		{}
	};

https://github.com/linux-test-project/ltp/tree/master/testcases/kernel/syscalls/statx/statx06.c

static struct tst_test test = {
	...
	.filesystems = (struct tst_fs[]) {
		{
			.type = "ext4",
			.mkfs_opts = (const char *const []){"-I", "256", NULL},
			.mnt_flags = MS_STRICTATIME,
		},
		{}

https://github.com/linux-test-project/ltp/tree/master/testcases/kernel/syscalls/quotactl/quotactl03.c

static struct tst_test test = {
	...
	.filesystems = (struct tst_fs []) {
		{
			.type = "xfs",
			.mnt_data = "usrquota",
		},
		{}
	},

https://github.com/linux-test-project/ltp/tree/master/testcases/kernel/syscalls/mmap/mmap16.c

static struct tst_test test = {
	...
	.filesystems = (struct tst_fs []) {
		{
			.type = "ext4",
			.mkfs_opts = (const char *const[]){
				"-b", "1024", NULL
			},
			.mkfs_size_opt = "10240",
		},
		{}
	},

[coolgw]

checking

[coolgw]

https://patchwork.ozlabs.org/project/ltp/patch/20250529105156.2564268-1-wegao@suse.com/