Update native pbc test files

This change updates native pbc test files and thus removes several TODO warnings in the test suite whereby the pbc files had an old version (e.g. TODO t/native_pbc/string_4_le.pbc has old v5.0.0.).  The files were updated with `tools/dev/pbc_header.pl` as recommended in the test output.  The platform used for this update was Debian Linux (wheezy) x86_64 (just in case that's relevant).

Comments most certainly welcome.