pg_amcheck: Try to fix still more test failures.

pg_amcheck: Try to fix still more test failures.

Avoid use of non-portable option ordering in command_checks_all().
The use of bare command line arguments before switches doesn't work
everywhere.  Per buildfarm members drongo and hoverfly.

Avoid testing for the message "role \"%s\" does not exist", because
some buildfarm machines report a different error. fairywren complains
about "SSPI authentication failed for user \"%s\"", for example.

Mark Dilger

Discussion: http://postgr.es/m/9E76E46A-48B2-4869-BD0C-422204C1F767@enterprisedb.com
Discussion: http://postgr.es/m/F0A1FD70-A2F4-4528-8A03-8650CAEC0554%40enterprisedb.com