Merge branch 'master' into ng/master

Merge branch 'master' into ng/master

This merge fixes automake bug#11791 for Automake-NG as well.

* master:
  tests: don't trust the exit status of "make -k" for non-GNU makes
  parallel-tests: reimplement fix for bug#11791
  tests setup: unset CDPATH
  tests setup: more namespace safeness
  tests setup: remove an unused variable
  tests setup: less hard-coding of the test subdirectory

+ Extra non-trivial edits:

* lib/am/parallel-tests.am: Manually merge the new implementation of the
fix for the bug#11791, as implemented by commit 'v1.12.1-95-gd5443e4' of
mainline Automake.
* Makefile.am (XFAIL_TESTS): Remove 't/parallel-tests-recheck-pr11791.sh'.
Add 't/test-missing.sh', which is slightly broken by this patch.  A later
patch will restore the lost behaviour.

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