[BUG] t/zzz-check-breaks.t requires a bunch of non-5.8-core modules

[ribasushi]

These lines can not work on a "clean room" perl: https://github.com/Test-More/test-more/blob/master/t/zzz-check-breaks.t#L7-L9

rabbit@Ahasver:~$ corelist CPAN::Meta
CPAN::Meta was first released with perl v5.13.10

rabbit@Ahasver:~$ corelist CPAN::Meta::Requirements
CPAN::Meta::Requirements was first released with perl v5.15.7

rabbit@Ahasver:~$ corelist Module::Metadata
Module::Metadata was first released with perl v5.13.9

[karenetheridge]

I'd do:

s/use CPAN::Meta;/eval { require CPAN::Meta; 1 } or plan skip_all 'breakage test requires CPAN::Meta';/

etc.

[karenetheridge]

I added another commit, to fall back to META.json if MYMETA.json does not exist.