Say so when make check runs no tests at all

Every test Makefile.am gates TESTS on GTEST_AVAILABLE, so --enable-tests
without --with-gtest compiles out all 38 suites and automake still prints
"All tests passed!" and exits 0. Warn at configure time and print a banner
from check-local, which runs after the recursion because "." is not in
SUBDIRS. Silent when gtest is present, and when tests were never requested.

Co-Authored-By: Claude <noreply@anthropic.com>