versioned perl5 coretests

For the B::C compiler, but eventually also for other implementations.
Allows parallel testing and failure tracking.

See rurban/perl-compiler#332 for the specs
and https://github.com/rakudo-p5/roast5 for a different perl6-style testsuite.

Here we just copy over the full p5p core tests per major important version
and add a directory to drive the tests. C-COMPILED is for the B::C/perlcc tests.
The idea to add this as git submodule to your testsuite, similar to the perl6
spectests.

Some test sources needed to be adjusted to work across multiple implementations
and versions. The eventual goal is to have only one latest version and symlink
old versions to it, back to 5.6.2. For now we keep the versioned directories.

The original p5p core testsuite works only 100% with p5p perl5, but not with a
compiled perl, divided into seperate compile-time and run-time phases, and not with
other implementations, like cperl, perlito, rakudo-p5 or p2.
This version overrides the handling of subtests and shelling out to call the
executable via run_perl.

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