tests: new requirement 'grep-nonprint'

tests: new requirement 'grep-nonprint'

* t/ax/test-inist.sh ($esc): New, a literal escape character.
(grep-nonprint): New requirement, check that the grep implementation
available that can handle non-printing characters correctly.
* t/color.sh: Use it instead of hand-rolled equivalent, and do not
(re)define '$esc' explicitly.
* t/color2.sh: Likewise.
* t/tap-color.sh: Likewise.
* t/vtexi4.sh: Likewise.
* t/parallel-tests-no-color-in-log.sh: Likewise.  Also prefer the use
of grep over that of $FGREP, because the new requirement only check
grep, and ensure we only grep non-printing characters from a pipe, to
avoid hitting a BSD grep limitation.
* t/parallel-tests-reset-term.sh: Likewise.
* t/ax/tap-summary-aux.sh: Use '$esc' instead of hard-coding the
literal escape character.

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