Fix tests for pt-upgrade

$ prove -vw --trap --timer t/pt-upgrade
...
[20:03:14] t/pt-upgrade/diff_rows.t ......... 
ok 1 - No diff
not ok 2 - 2 diffs (ORDER BY ASC vs. DESC)

#   Failed test '2 diffs (ORDER BY ASC vs. DESC)'
#   at t/pt-upgrade/diff_rows.t line 52.
#     Structures begin differing at:
#          $got->[1][1][0] = 'mysql.session'
#     $expected->[1][1][0] = 'root'
# $VAR1 = [
#   [
#     1,
#     [
#       'msandbox'
#     ],
#     [
#       'root'
#     ]
#   ],
#   [
#     2,
#     [
#       'mysql.session'
#     ],
#     [
#       'mysql.sys'
#     ]
#   ],
#   [
#     3,
#     [
#       'mysql.sys'
#     ],
#     [
#       'mysql.session'
#     ]
#   ]
# ];
# >> 3
not ok 3 - Host1 missing a row

#   Failed test 'Host1 missing a row'
#   at t/pt-upgrade/diff_rows.t line 52.
#     Structures begin differing at:
#          $got->[0][0] = '3'
#     $expected->[0][0] = '1'
# $VAR1 = [
#   [
#     3,
#     undef,
#     [
#       [
#         'mysql.session'
#       ],
#       [
#         'mysql.sys'
#       ],
#       [
#         'root'
#       ]
#     ]
#   ]
# ];
not ok 4 - Host2 missing a row

#   Failed test 'Host2 missing a row'
#   at t/pt-upgrade/diff_rows.t line 52.
#     Structures begin differing at:
#          $got->[0][0] = '3'
#     $expected->[0][0] = '1'
# $VAR1 = [
#   [
#     3,
#     [
#       [
#         'mysql.session'
#       ],
#       [
#         'mysql.sys'
#       ],
#       [
#         'root'
#       ]
#     ],
#     undef
#   ]
# ];
ok 5 - Bug 1168434: no diff with NULL
ok 6 - Sandbox servers
1..6
# Looks like you failed 3 tests of 6.
Dubious, test returned 3 (wstat 768, 0x300)
Failed 3/6 subtests