tests: tighten checks for octal escapes in shell printf. printf on AIX default shell ksh (89) says \1 is an invalid escape. It insists on at least 2 digits. This causes failures in test-keyword.t and test-status.t. check-code.py already looks out for \NNN and recommends using Python for outputting octal values. Extend the check to \NN and \N and fix up resulting failures.