DejaGnu unit testing protocol

I had been planning to look at this later, but fixing PR42399 brought 
this front-and-center.  The DejaGnu unit testing protocol is not (yet) 
documented (will be fixed as part of PR42399) and there is a strange 
quirk in its handler.

What produces lines with prefixes matching {[0-9][0-9]:..:..:} and/or 
why is host_execute looking for that pattern?  These lines have the 
effect only of resetting a timeout counter and printing the line if 
verbose is 3 or higher but are otherwise ignored.


The DejaGnu unit testing protocol is very simple, and I think I can work 
with this fairly well for the long-term.  DejaGnu spawns a unit test 
program and reads that program's output.  Arguments for the unit test 
program can be specified in the testsuite.  The unit test program can 
produce any output, but DejaGnu is sensitive to lines beginning with a 
tab character, followed by uppercase letters, followed by a colon.  
These lines (currently) indicate test results and (in the future) 
provide additional information to effect proper testing.  (Examples of 
future extensions now planned include support for known failures and 
framework-assessed expected failures.  Currently, the dejagnu.h header 
provides xfail/xpass functions to allow unit tests to declare expected 
failure, but host_execute does not recognize these results; this second 
bug is now slated to be fixed for 1.6.3.)

Unit test programs may also produce other output for the benefit of a 
developer directly running them or reading the DejaGnu log, but the 
sequence {\n\t[][[:upper:]]*:} should be considered reserved for the 
DejaGnu unit testing support.


Anyone know why that pattern was included in host_execute or have other 
comments?


-- Jacob