tests: Use "command -v" instead of which(1) in shell scripts

When which(1) is not installed, we would complain "perl not found"
because it's the first set_prog_path check. The error message is
wrong.

Fix it by using "command -v", a native way to query the existence of a
command.

Suggested-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 1416380832-9697-1-git-send-email-famz@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
