Fix tests for pt-config-diff

$ prove -vw --trap --timer t/pt-config-diff/
...
[17:31:50] t/pt-config-diff/basics.t ..... 
ok 1 - Server active config doesn't differ with itself
ok 2 - No output when no diff
ok 3 - Exit 1 when diffs found
ok 4 - Diff output
not ok 5 - my.sandbox.cnf doesn't differ with active config

#   Failed test 'my.sandbox.cnf doesn't differ with active config'
#   at t/pt-config-diff/basics.t line 90.
#          got: '1'
#     expected: '0'
not ok 6 - No output

#   Failed test 'No output'
#   at t/pt-config-diff/basics.t line 96.
#          got: '1 config difference
# Variable                  /tmp/12345/my.sandbox.cnf Delly-7390
# ========================= ========================= ==========
# innodb_buffer_pool_size   16777216                  2147483648
# '
#     expected: ''
...
# Looks like you failed 2 tests of 15.
Dubious, test returned 2 (wstat 512, 0x200)
Failed 2/15 subtests