[testasync] hangs

Starting with checkin

http://core.tcl.tk/tcl/info/861e1fd6e9

a --enable-threads build of the core-8-4-branch
hangs in async.test and in the test parse-8.8
because the [testasync] command blocks.

This should be corrected before a final 8.4.20 can
get released.

A simple-minded correction would be to revert this
commit, but you may have a better idea.

Thanks for looking!