test-cutils: Refactor qemu_strtosz tests for less boilerplate

test-cutils: Refactor qemu_strtosz tests for less boilerplate

No need to copy-and-paste lots of boilerplate per string tested, when
we can consolidate that behind helper functions.  Plus, this adds a
bit more coverage (we now test all strings both with and without
endptr, whereas before some tests skipped the NULL endptr case), which
exposed a SEGFAULT on qemu_strtosz(NULL, NULL, &val) that will be
fixed in an upcoming patch.

Note that duplicating boilerplate has one advantage lost here - a
failed test tells you which line number failed; but a helper function
does not show the call stack that reached the failure.  Since we call
the helper more than once within many of the "unit tests", even the
unit test name doesn't point out which call is failing.  But that only
matters when tests fail (they normally pass); at which point I'm
debugging the failures under gdb anyways, so I'm not too worried about
it.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Hanna Czenczek <hreitz@redhat.com>
Message-Id: <20230522190441.64278-12-eblake@redhat.com>