Don't look for test info inside .tesla files.

Unfortunately protobufs don't let us stick comments into their textual representation, so we can't describe tests in them.
Make ArgString, DotName, etc. accept NULL.

If the value is NULL, return the string "NULL" (or a useful equivalent). This simplifies exceptional-case checking (*i ? ShortName(**i) : "X") elsewhere and lets us be consistent about how we represent NULL.
More readable (and testable!) .dot output.