Merge branch 'testsuite-refactor' into maint

* testsuite-refactor: (33 commits)
  maintcheck: fixup list of files in $(xdefs)
  tests: never source test-defs.sh directly, source test-lib.sh instead
  runtest: sanitize test environment
  tests: remove an obsolescent self test
  tests: "am_using_tap=yes" -> "am_test_protocol=tap"
  tests: protect test libs against multiple inclusion
  configure: testsuite shell can return early from "dot-sourced" files
  tests: move sanitization and "Bournification" in the generic test lib
  tests: source test defs in the generic test lib
  test defs: no need to re-add $srcdir/t/ax to $PATH
  tests: split test libs into "generic" and "automake-specific"
  test setup: move actual calling of testsuite setup in ./defs
  test setup: merge definitions of function for simple tests
  test init: refactor: new function 'am_test_setup'
  test init: refactor: move displaying of debugging info later
  test init: refactor: new function 'am_setup_testdir'
  test init: refactor: new function 'am_set_exit_traps'
  configure: testsuite shell set exit traps in shell functions
  test init: refactor: new function 'am_exit_trap'
  test init: refactor: new function 'process_requirements'
  ...