Avoid odd portability problem in TestLib.pm's slurp_file function.

Avoid odd portability problem in TestLib.pm's slurp_file function.

For unclear reasons, this function doesn't always read the expected data
in some old Perl versions.  Rewriting it to avoid use of ARGV seems to
dodge the problem, and this version is clearer anyway if you ask me.

In passing, also improve error message in adjacent append_to_file function.