Add $meta{prereqs}{test} to MYMETA.json and META.json

Module::Build 0.4004 supports test_configure. But MYMETA.json and META.json (generated by `perl ./Build.PL` and `./Build distmeta`) does not have $meta{prereqs}{test}. They have x_test_requires.

After ./Build distmata, When I run `perl ./Build.PL` again. I got $meta{prereqs}{test} in MYMETA.json. 
It is enough for installing prereqs modules, but confusing. I want to get $meta{prereqs}{test} in first `perl ./Build.PL`

This patch upgrades a metadata that created by get_metadata() to spec-2 before write meta files.