Prevent ExtUtils-CBuilder leaving test output on Windows

The link function call in the have_compiler and have_cplusplus tests create
a compilet.def file on Windows which is correctly recorded for cleaning up
when the EU::CB object is destroyed, but if another one gets made in the
meantime then ExtUtils::Mksymlists::Mksymlists moves the first one to
compilet_def.old, which isn't recorded for cleaning up and gets left
behind when the test script has finished. Using a new object each time,
destroying the previous one first, prevents this.
Stop t/io/utf8.t leaving a temporary file behind on Windows

Filehandles need to be closed for unlink to work on Windows.
Stop t/op/coreamp.t leaving temporary files behind

test.pl will delete any file made by tempfile(), but it won't delete the
*.dir and *.pag files made by dbmopen. Hopefully this is the right way to
delete them, cribbed from lib/AnyDBM_File.t.