check: rename AM_TESTS_SETUP -> AM_TESTS_ENVIRONMENT

check: rename AM_TESTS_SETUP -> AM_TESTS_ENVIRONMENT

The AM_TESTS_SETUP naming was not a good one after all.  It may
be technically more correct than AM_TESTS_ENVIRONMENT, but the
latter is a better one simply because it is easier to remember,
and even if you've never heard of it and only know the semantics
of TESTS_ENVIRONMENT, you can have a straightforward way to
figure out how AM_TESTS_ENVIRONMENT would work.

* tests/check.am (am__check_pre): Update.
* doc/automake.in (Simple Tests using parallel-tests): Update.
* tests/parallel-tests-am_tests_setup.test: Renamed ...
* tests/parallel-tests-am_tests_environment.test: ... to this,
and updated.
* tests/Makefile.am (TESTS): Update.

Suggestion and motivation by Ralf Wildenhues.