Extend testsuite.yml to 'select' which jobs to run

For the Perl/perl5 repository nothing changes.

For a fork of perl5 the user now has some more control to select which
jobs to run. This is done by setting the appropriate secrets.

Before this commit one could choose to:
- run no tests (= GitHub Actions disabled)
- run 'Sanity Check' (= GitHub Actions enabled)
- run all jobs (= GitHub Actions enabled + EXTENDED_TESTING secret added)

With this commit specific jobs can be enabled by adding a secret
with a true value.

For example: setting `CI_FORCE_CYGWIN=1` in the Secrets will cause it to
run the 'Sanity Check' job and the Cygwin' job. All other jobs are skipped.

(See the docs in the file to see the full list of Secrets)

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