conformtest: Support xfail markers on individual assertions [committed]

[Roland McGrath]

I like the intent.  I noticed that almost all of the instances in the -data
files use a consistent convention for the comment:
	// Bug NNNNN: ...

My first impulse was to ask that the convention be stated somewhere and
followed uniformly.  But it occurs to me it would be nice to have this be
more formal than a comment convention.  How about instead of:
	// Bug NNNNN: explanatory comment
	xfail-foo ...
we make it:
	// explanatory comment
	xfail {NNNNN} foo ...

This makes it completely explicit that you can't have an XFAIL without
having a bug filed for it.  It also makes it marginally easier than a
comment convention to have scripts that do something or other with the bug
number.


Thanks,
Roland