Move release tests into t/release/. Don't run as part of 'make test'.

[schwern]

There's a bunch of useful-for-development tests in t/porting, and a bunch of stuff that we only care about when making a release. Developers running make test and Travis should not run release tests. Developers should not have to care about things like the AUTHORS or MANIFEST files.

• Move the unwanted tests into t/release/.
• Don't run t/release/ as part of make test (this should happen by default).
• Add make test_release which runs tests in t/release/ and nothing else. (See test_porting in Makefile)

What should go into t/release? At least these. Normal developers should not have to manage the AUTHORS nor MANIFEST files.

• t/porting/authoring.t
• t/porting/manifest.t