Merge branch 'ad-parallel-tests' into next

Merge branch 'ad-parallel-tests' into next

* ad-parallel-tests:
  Fix LAZY_TEST_SUITE handling and $(TEST_SUITE_LOG) recreation.
  Fix parallel-tests with empty $(TESTS) for BSD make.
  Mark check-html and check-TESTS as recursive for BSD make.
  Match XFAIL_TESTS correctly with Solaris make.
  Use append mode to capture parallel test output.
  parallel-tests: Fix summary output.
  Overhaul of tests/README.
  Documentation for the parallel-tests driver.
  New tests for `parallel-tests'.
  parallel-tests: Ensure backward-compatible semantics.
  Parallel test execution: new option `parallel-tests'.