[PROPOSAL] TAP test example

Don't we need a simple example of TAP tests in src/test ? Something that test
a trivial feature, but shows basic testing tricks?

While explaining to my friends how does TAP test works I wrote an example TAP
test. May be we we should add it to the core with sensible README explanation?

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Nikolay Shaplov
Postgres Professional: http://www.postgrespro.com
Russian Postgres Company