Fix tests for pt-find

$ prove -vw --trap --timer t/pt-fifo-split/
...
[17:39:45] t/pt-fifo-split/pt-fifo-split.t .. 
ok 1 - It lives
ok 2 - --lines=2 with 9 lines works as expected
ok 3 - --lines=15 with 9 lines works as expected
ok 4 - I read the file
ok 5 - Offset works
not ok 6 - Dies if PID file already exists (issue 391)

#   Failed test 'Dies if PID file already exists (issue 391)'
#   at t/pt-fifo-split/pt-fifo-split.t line 154.
#                   'Can't locate object method "make_PID_file" via package "Daemon" at /home/sveta/src/percona-toolkit/bin/pt-fifo-split line 1426.
# '
#     doesn't match '(?^:PID file /tmp/pt-fifo-split.pid.107997 already exists)'
1..6
# Looks like you failed 1 test of 6.
Dubious, test returned 1 (wstat 256, 0x100)
Failed 1/6 subtests