Don't create a RUN-PACKAGE-TESTS in every package

* README.md: Adjust README.md to new RUN-PACKAGE-TESTS and tweak
it.

* test/package.lisp (defpackage :stefil-self-tests): Renamed from
:STEFIL-TESTS for clarity.

* source/suite.lisp (define-test-package): Don't define
RUN-PACKAGE-TESTS function in target package.
(run-package-tests): New exported function. Takes a PACKAGE kwarg that
defaults to *PACKAGE*
(run-suite-tests): Take a SUITE-DESIGNATOR instead of a SUITE-SYM.

* test/basic.lisp (depackage :stefil-self-tests): Renamed from
:STEFIL-TESTS for clarity.

* test/intro-example.lisp: New file.

* test/stefil-test.asd (asdf:defsystem :stefil-self-tests): Add
"intro-example.lisp"