tests: fix spurious error when run directly with Solaris /usr/xpg4/bin/sh

tests: fix spurious error when run directly with Solaris /usr/xpg4/bin/sh

If any test script is run directly with Solaris 10 /usr/xpg4/bin/sh or
/bin/ksh, it fails spuriously with a message like:

  t0000-basic.sh[31]: unset: bad argument count

This happens because those shells bail out when encountering a call to
"unset" with no arguments, and such unset call could take place in
'test-lib.sh'.  Fix that issue, and add a proper comment to ensure we
don't regress in this respect.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>