Doc: improve README files associated with TAP tests.

Doc: improve README files associated with TAP tests.

Rearrange src/test/perl/README so that the first section is more
clearly "how to run these tests", and the rest "how to write new
tests".  Add some basic info there about debugging test failures.
Then, add cross-refs to that READNE from other READMEs that
describe how to run TAP tests.

Per suggestion from Kevin Burke, though this is not his original
patch.

Discussion: https://postgr.es/m/CAKcy5eiSbwiQnmCfnOnDCVC7B8fYyev3E=6pvvECP9pLE-Fcuw@mail.gmail.com