Add several more subroutines for test logging.

• log_debug: Only print the string if $STF_DEBUG is set.
• log_mustbe: Like log_must, but the first argument specifies the

	  required exit code; since its behavior differs from log_must in
	  certain ways that assume a success exit code of zero, it is not used
	  by log_must.

• log_cmd: Log the given command and its exit code, but let the caller

	  decide how to deal with the returned exit code.

• log_onfail: Like log_cmd, but only logs on failure.

Submitted by: Will
Sponsored by: Spectra Logic Corp