Add cachestat() test

[metan-ucw]

There is a selftest:

commit 88537aac0b3840314d3a5cc401a7fc400c069bc9
Author: Nhat Pham <nphamcs@gmail.com>
Date:   Tue May 2 18:36:08 2023 -0700

    selftests: add selftests for cachestat

With some fixes on the top:

commit f84f62e69963d7742acec4340ec1c4c7ef22b887
Author: Andre Przywara <andre.przywara@arm.com>
Date:   Mon Aug 21 17:05:34 2023 +0100

    selftests: cachestat: catch failing fsync test on tmpfs

[coolgw]

checking

[coolgw]

https://patchwork.ozlabs.org/project/ltp/patch/20230908072925.15102-1-wegao@suse.com/

[acerv]

I simplified the patches a bit to diverge from kselftests which are adding too much complexity in my opinion https://patchwork.ozlabs.org/project/ltp/list/?series=407039

[coolgw]

https://patchwork.ozlabs.org/project/ltp/patch/20230908072925.15102-1-wegao@suse.com/
Hold on v2 currently, wait acerv's new patch.