perl-module.eclass: Rename SRC_TEST to DIST_TEST in EAPI=6, default to "do parallel...

perl-module.eclass: Rename SRC_TEST to DIST_TEST in EAPI=6, default to "do parallel", add features

* The variable SRC_TEST used to allow some control over the testing phase (run
it, run tests parallelized, skip it). Named DIST_TEST in EAPI=6 now for
consistency.

* Add functionality for ebuild authors: even if the ebuild specifies that e.g.
tests should not be run or run single-threaded, allow in EAPI=6 to override
that from make.conf or package.env.

* Rewrite of the test phase for EAPI=6. Adds options to control test verbosity
and switch network tests (something that might or might not work depending on
upstream cooperativity).