Fix paragraph confusion in pod/perlhack.pod

Fix paragraph confusion in pod/perlhack.pod

As far as I understand the diffs, this problem occured first with commit
a75f557cc482d1753211634e8aac8b7572677015: The section about "Parallel
tests" had been inserted into the list about "Special Make Test Targets"
before the last "=item" of the list, but without an empty line between
the two of them.

The Great Revision of commit 04c692a854b61dfae1266e29468ce4fb51c80512
turned the "=item" about "Parallel tests" into a "=head2", leaving the
"=item" statement without the empty line in its old place.

There may have been some more editing in this corner. However, I
rearranged the lines, so that the "=item" statement goes with its original
list about the make test targets again.