tests: fix signo to signame lookup in opr/softsig tests

tests: fix signo to signame lookup in opr/softsig tests

Fix the loop condition when scanning the signal number to name table to
convert a signal number to a name.  Instead of looping sizeof(size_t)
times, loop for the number of elements in the table.

This bug was masked on 64 bit-platforms, since the signal number to name
table table currently has 8 elements, which is coincidently the same as
sizeof(size_t) on 64-bit platforms.  The bug becomes apparent on 32-bit
systems; only the first 4 elements of the table are checked.

Example error output before this fix:

    $ cd tests
    $ ./libwrap ../lib ./runtests -o opr/softsig
    1..11
    ok 1
    ok 2
    ok 3
    ok 4
    ok 5
    not ok 6
    # Failed test in ./opr/softsig-t at line 57.
    # got: 'Received UNK
    # '
    # expected: 'Received TERM
    # '
    not ok 7
    # Failed test in ./opr/softsig-t at line 60.
    # got: 'Received UNK
    # '
    # expected: 'Received USR1
    # '
    not ok 8
    # Failed test in ./opr/softsig-t at line 63.
    # got: 'Received UNK
    # '
    # expected: 'Received USR2
    # '
    ok 9 - Helper exited on KILL signal.
    ok 10 - Helper exited on SEGV signal.
    ok 11 # skip Skipping buserror test; SIGBUS constant is not defined.
    # Looks like you failed 3 tests of 11.

Change-Id: I863cc9f3650c4a5e9ac9159d90e063b986a8460a
Reviewed-on: https://gerrit.openafs.org/12367
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>