Use PERLPOWERTOOLS_PROGRAMS to decide what to test

There are two test situations that I want to make work.

1. Test everything
2. Test what I'm working on write now

By default, the tests run all the checks on all of
the programs under bin/ (other than perlpowertools and perldoc).

To run tests for a particular program, set the 
PERLPOWERTOOLS_PROGRAMS env var to the basename of
the programs you want:

env PERLPOWERTOOLS_PROGRAMS=mimedecode perl t/000.basic.t

env PERLPOWERTOOLS_PROGRAMS=mimedecode,whois perl t/000.basic.t

This is incomplete because we still need a way to 
go through all of t/ and skip directories that don't correspond to the programs we want to limit 
the test tests to.

CONTRIBUTING.md

@@ -34,7 +34,7 @@ things you need to do:
 * Run `make manifest` and check that it adds your new command and tests
 
 For new programs, include tests with as high a coverage as you can
-stand.
+stand. Each program should have its own directory under *t/*.
 
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