Fix tests for pt-show-grants

$ prove -vw --trap --timer t/pt-show-grants
...
[19:04:48] t/pt-show-grants/standard_options.t .. 
1..1
not ok 1 - Dies if PID file already exists (issue 391)

#   Failed test 'Dies if PID file already exists (issue 391)'
#   at t/pt-show-grants/standard_options.t line 24.
#                   'Can't locate object method "make_PID_file" via package "Daemon" at /home/sveta/src/percona-toolkit/bin/pt-show-grants line 1924.
# '
#     doesn't match '(?^:PID file /tmp/mk-script.pid already exists)'
# Looks like you failed 1 test of 1.
Dubious, test returned 1 (wstat 256, 0x100)
Failed 1/1 subtests 

...

Test Summary Report
-------------------
t/pt-show-grants/all_grants.t      (Wstat: 512 Tests: 3 Failed: 2)
  Failed tests:  1-2
  Non-zero exit status: 2
t/pt-show-grants/basics.t          (Wstat: 1024 Tests: 15 Failed: 4)
  Failed tests:  11-14
  Non-zero exit status: 4
Files=4, Tests=24,  7 wallclock secs ( 0.04 usr  0.02 sys +  1.87 cusr  1.31 csys =  3.24 CPU)
Result: FAIL