selftests: add a sanity check for zswap

We recently encountered a bug that makes all zswap store attempt fail. 
Specifically, after:

"141fdeececb3 mm/zswap: delay the initialization of zswap"

if we build a kernel with zswap disabled by default, then enabled after
the swapfile is set up, the zswap tree will not be initialized.  As a
result, all zswap store calls will be short-circuited.  We have to perform
another swapon to get zswap working properly again.

Fortunately, this issue has since been fixed by the patch that kills
frontswap:

"42c06a0e8ebe mm: kill frontswap"

which performs zswap_swapon() unconditionally, i.e always initializing
the zswap tree.

This test add a sanity check that ensure zswap storing works as
intended.

Link: https://lkml.kernel.org/r/20231020222009.2358953-1-nphamcs@gmail.com
Signed-off-by: Nhat Pham <nphamcs@gmail.com>
Acked-by: Rik van Riel <riel@surriel.com>
Cc: Domenico Cerasuolo <cerasuolodomenico@gmail.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Zefan Li <lizefan.x@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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