tools/testing: keep legacy generated files around in .gitignore file

tools/testing: keep legacy generated files around in .gitignore file

People keep removing generated files from .gitignore files even when the
files stay around.  Please don't do that: just because the file is no
longer being generated doesn't make it magically go away, and doesn't
make it suddenly be something that should now not be ignored any more.

Fixes: dd2c6ec24fca ("selftests/mm: remove virtual_address_range test")
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: SeongJae Park <sj@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>