tests: trace_ui followed a signal that was not one (and it really did fail)

"the code view follows the trace" failed on the port (None vs 0x2ae4) and
passed on master, but it was not a trace regression: the test pressed "]",
waited for `app._t == 1`, and then read the listing cursor. `app._t` is
assigned the moment the key is handled -- the navigation it starts runs in a
worker -- so the wait was satisfied before the view had moved, and the check
read a cursor that had no address yet.

Master won that race because its backend answers in single-digit milliseconds.
The Code Mode backend is slower, so the race became a reliable failure. The
gate is now the condition the check is about (the cursor is on the trace's ip),
for both the forward and the backward step.

39 passed, 0 failed -- the same tally as master.

0 files changed, 0 insertions, 0 deletions