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 <efd5b15649c943c26fb76b69835a0b145831b6a9@gmail.com>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git