test-hash: Do not exit check_word_hash() when there is a failure.

This commit makes check_word_hash() run to finish even when there
is a failure during the run.  The test will still fail due to the
output check in AT_CHECK.  And developers can benefit from having
all failed hashes instead of only the first one.

Signed-off-by: Alex Wang <alexw@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>