tests/README: update obsoleted advice

tests/README: update obsoleted advice

* tests/README (Section "Writing test cases" subsection "Do"):
Do not suggest to use the `*-p.test' pattern for the names of
hand-written tests which use the `parallel-tests' Automake option.
Not only is this not respected by the existing tests, but it is
more likely to cause conflicts with auto-generated tests.
So, suggest to *avoid* using the `*-p.test' pattern in names
of hand-written tests instead.
(Section "Writing test cases" subsection "Do not"):  When
suggesting not to override Makefile variables using command
line arguments, do not use the badly outdated variables `U'
and 'ANSI2KNR' in the example; instead, use the more common
and typical `DESTDIR'.