[perl #121337] fix issues with op/utf8cache.t

This test had two problems:

- it used Devel::Peek::Dump() without loading it, broken when the script
  was refactored to use test.pl in c45bec600

- the C< open CHILD "-|" > doesn't work on Win32

Since the test didn't check it was receiving reasonable dump output,
neither problem resulted in a test failure.