Need t/spectest.data.6.d

[zoffixznet]

In Rakudo's repo, we have several files that list which roast test files the test suite needs to run. If 6.c roast is being tested, t/spectest.data.6.c will be used. And by default, t/spectest.data will be used, which is typically used by roast master branch, which contains proposals for the next language version.

Since 6.d spec was released, we need to create t/spectest.data.6.d file, based on current t/spectest.data.

There are some docs for how this system works: https://github.com/rakudo/rakudo/blob/master/docs/roast-spectest.data-versioning.md

Also, 6.d release forgot to update the VERSION file. Filed as Raku/roast#506

[vrurg]

With regard to Raku/problem-solving#31 discussion, revision-specific files are not needed. To mark milestones and revision statuses tags and branches are used: 6.<revision> tag on master as a watershed between previous and following PREVIEWs and 6.<revision>-errata branch for changes specific to that particular release.