This commit finishes the reorganization of the tests for the checker
`security.ArrayBound`.

Previously these tests were all named `out-of-bounds-*` which was only
weakly connected to the checker name; this commit moves them to a
directory named after the checker (`ArrayBound`). I decided to use a
directory instead of the more common filename prefix ("poor man's
directory") system because it seems to be a more natural use of the
filesystem and there are already a few precedents for it.

I also added (or edited) comments at the beginning of each test file to
describe their purpose; and I added a single new testcase to highlight
that the assumption note tags can be added to reports by any checker.
(Previously all tests in the file triggered out-of-bounds reports to
reveal the note tags; but that was just for convenience.)