[tlbuild] once again test build

[Karl Berry]

Hi everyone - Peter has imported the final ICU 53.1 release, and
luatex/mpost/xetex/etc. have been updated.  We don't expect further
significant changes in the sources, mainly reactions to (your) bug
reports.

So, please try another test build and report problems.

We are still in build-only mode: no installs into the bin/ dirs, please.
Unfortunately, I am going to be away most of next week (Wed-Mon). 
I expect we'll freeze TL13 and start the TL14 pretest shortly after I
return.

Thanks,
Karl