Avoid a spurious test failure with shells that ignore write errors.

Avoid a spurious test failure with shells that ignore write errors.

In a surprisingly large number of cases, the ‘printf’ shell utility
doesn’t report errors caused by writing to stdout.  This can cause
a spurious test failure on systems that have this bug in ‘printf’
and also have /dev/full.  Detect such systems and skip that subtest.

Problem reported by Alan Coopersmith.

* tests/torture.at (AC_CONFIG_FILES, HEADERS, LINKS and COMMANDS):
  If /bin/sh’s printf doesn’t report an error on writing to /dev/full,
  skip the subtests that write to /dev/full.