Merge branch 'master' into ng/master

Merge branch 'master' into ng/master

* master: (39 commits)
  typofix: in a comment in Makefile.am
  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'
  ...

+ Extra non-trivial edits:

* Makefile.am (perf_TESTS): Define here, as 't/list-of-tests.mk' is gone
in the 'ng/master' branch.
(all_TESTS): Don't list tests in 't/perf'; this reflects the removal of
$(perf_TESTS) from $(handwritten_TESTS) done in 't/list-of-tests.mk' on
the 'master' branch.

* t/depcomp-shuffle.sh: Source 'test-lib.sh', not './defs-static'.
Also, simply use:
    . depcomp-shuffle.sh
instead of:
    . "$am_testauxdir"/depcomp-shuffle.sh
to source the helper shell script 'depcomp-shuffle.sh'.  See recent
commit 'v1.12.2-49-g42fb45b' for a rationale.
* t/depcomp-shuffle-sub-vpath.sh: Likewise.
* t/depcomp-shuffle-sub.sh: Likewise.
* t/depcomp-shuffle-vpath.sh: Likewise.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>