tools/selftests: expand all guard region tests to file-backed

Extend the guard region tests to allow for test fixture variants for anon,
shmem, and local file files.

This allows us to assert that each of the expected behaviours of anonymous
memory also applies correctly to file-backed (both shmem and an a file
created locally in the current working directory) and thus asserts the
same correctness guarantees as all the remaining tests do.

The fixture teardown is now performed in the parent process rather than
child forked ones, meaning cleanup is always performed, including
unlinking any generated temporary files.

Additionally the variant fixture data type now contains an enum value
indicating the type of backing store and the mmap() invocation is
abstracted to allow for the mapping of whichever backing store the variant
is testing.

We adjust tests as necessary to account for the fact they may now
reference files rather than anonymous memory.

Link: https://lkml.kernel.org/r/ab42228d2bd9b8aa18e9faebcd5c88732a7e5820.1739469950.git.lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Jann Horn <[email protected]>
Cc: John Hubbard <[email protected]>
Cc: Kalesh Singh <[email protected]>
Cc: Liam Howlett <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: "Paul E . McKenney" <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: Suren Baghdasaryan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: lorenzo-stoakes