xarray: port tests to kunit

Minimally rewrite the XArray unit tests to use kunit.  This integrates
nicely with existing kunit tools which produce nicer human-readable output
compared to the existing machinery.

Running the xarray tests before this change requires an obscure
invocation

```
tools/testing/kunit/kunit.py run --arch arm64 --make_options LLVM=1 \
  --kconfig_add CONFIG_TEST_XARRAY=y --raw_output=all nothing
```

which on failure produces

```
BUG at check_reserve:513
...
XArray: 6782340 of 6782364 tests passed
```

and exits 0.

Running the xarray tests after this change requires a simpler invocation

```
tools/testing/kunit/kunit.py run --arch arm64 --make_options LLVM=1 \
  xarray
```

which on failure produces (colors omitted)

```
[09:50:53] ====================== check_reserve  ======================
[09:50:53] [FAILED] param-0
[09:50:53]     # check_reserve: EXPECTATION FAILED at lib/test_xarray.c:536
[09:50:53] xa_erase(xa, 12345678) != NULL
...
[09:50:53]     # module: test_xarray
[09:50:53] # xarray: pass:26 fail:3 skip:0 total:29
[09:50:53] # Totals: pass:28 fail:3 skip:0 total:31
[09:50:53] ===================== [FAILED] xarray ======================
```

and exits 1.

Use of richer kunit assertions is intentionally omitted to reduce the
scope of the change.

[akpm@linux-foundation.org: fix cocci warning]
  Link: https://lore.kernel.org/oe-kbuild-all/202412081700.YXB3vBbg-lkp@intel.com/
Link: https://lkml.kernel.org/r/20241205-xarray-kunit-port-v1-1-ee44bc7aa201@gmail.com


Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Cc: Bill Wendling <morbo@google.com>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Justin Stitt <justinstitt@google.com>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Naveen N Rao <naveen@kernel.org>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>