Merge branch 'maint'

Merge branch 'maint'

* maint:
  tests: avoid one last `...` command substitution in 'test-init.sh'
  maintcheck: guard against `...` for command substitution in test cases
  tests: more uses of $(...) over `...` for command substitution
  cosmetics: quote `like this', not 'like this', in a couple of tests
  readme: clarify/extend few entries in 't/README'
  readme: better separation of entries in 't/README'
  readme: subsections "Do" and "Do not" in 't/README' merged
  tests: new requirement 'grep-nonprint'
  cosmetics: fix description of an expected error message in a test
  tests: assume automake quotes 'like this', not `like this'

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