document t/test.pl

I got sick of reverse engineering run_multiple_progs() tests and
runperl() arguments each time I needed to use them.

There's some documentation in comments inline but it's pretty variable
and less accessible than pod.

Since (in theory anyway) we want test.pl to exercise as little of perl
as possible, the POD doesn't belong in test.pl itself.

So I've put this in t/test_pl.pod since it's not really end user
documentation that would belong in pod/.

Author: tonycoz

MANIFEST

@@ -6682,9 +6682,11 @@ t/run/switchx3.aux			Data for switchx.t
 t/run/todo.t				TODO tests
 t/TEST					The regression tester
 t/test.pl				Simple testing library
+t/test_pl.pod				Documentation for test.pl
 t/test_pl/_num_to_alpha.t		Tests for the simple testing library
 t/test_pl/can_isa_ok.t			Tests for the simple testing library
 t/test_pl/display.t			Tests for the simple testing library
+t/test_pl/examples.t			Tests for the simple testing library
 t/test_pl/plan_skip_all.t		Tests for the simple testing library
 t/test_pl/tempfile.t			Tests for the simple testing library
 t/thread_it.pl				Run regression tests in a new thread