tests: fix false failure due to race

tests: fix false failure due to race

* tests/misc/tty-eof.pl: Fix a race that commit v9.10-269-gf312af49a
made more apparent, and resulted in intermittent failure like:
"... (with input) didn't produce expected output".
Now Instead of waiting for overlapping echo and output,
just wait for EOF, and if received, use exp->before()
to inspect the accumulated output.