tests: fix an ls test not to fail when user or group name contains SP

* tests/ls/block-size.sh (size_etc): The sed expression through which
we filtered the output of "ls -l ..." assumed that the user and group
name components of each line would not contain spaces.  Avoid the
problem by using -og instead of -l, thus not printing either of those
fields.  Adjust the sed expression accordingly.
tests: avoid spurious failure when SIZE_MAX is 2^32-1

* tests/split/line-bytes.sh: Since we've limited virtual memory to
20MB, choose a smaller size, 1GiB (which is <= SIZE_MAX) rather than
1EiB, which is larger than SIZE_MAX on 32-bit systems.
I confirmed that this test still fails when the split.c-modifying
part of v8.21-58-gfec363c is backed out.