tools/testing/selftests: assert that anon merge cases behave as expected

Prior to the recently applied commit that permits this merge,
mprotect()'ing a faulted VMA, adjacent to an unfaulted VMA, such that the
two share characteristics would fail to merge due to what appear to be
unintended consequences of commit 965f55dea0e3 ("mmap: avoid merging
cloned VMAs").

Now we have fixed this bug, assert that we can indeed merge anonymous VMAs
this way.

Also assert that forked source/target VMAs are equally rejected. 
Previously, all empty target anon merges with one VMA faulted and the
other unfaulted would be rejected incorrectly, now we ensure that unforked
merge, but forked do not.

Additionally, add the new test file to the MEMORY MAPPING section in
MAINTAINERS, as these tests are explicitly memory mapping related.

Link: https://lkml.kernel.org/r/2b69330274a3b71721f7042c5eabe91143934415.1744104124.git.lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Yeoreum Yun <yeoreum.yun@arm.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Jann Horn <jannh@google.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Rik van Riel <riel@surriel.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Wei Yang <richard.weiyang@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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