[gdb/testsuite] Make tui testing less verbose

Currently, tui testing is rather verbose.  When using these RUNTESTFLAGS to
pick up all tui tests (17 in total):
...
rtf=$(echo $(cd src/gdb/testsuite/; find gdb.* -type f -name *.exp* \
  | xargs grep -l tuiterm_env) )
...
we have:
...
$ wc -l gdb.log
120592 gdb.log
...

Most of the output is related to controlling the tui screen, but that does
not give a top-level sense of how the test-case progresses.

Put differently: a lot of bandwith is used to describe how we arrive at a
certain tui screen state.  But we don't actually always show the state we
arrive at, unless there's a FAIL.

And if there's say, a PASS that should actually be FAILing, it's hard to
detect.

Fix this by:
- dropping the -log on the call to verbose in _log.  We still can get the
  same info back using runtest -v.
- dumping the screen or box that we're checking, also when the test passes.

Brings down verbosity to something more reasonable:
...
$ wc -l gdb.log
3221 gdb.log
...

Tested on x86_64-linux.