[gdb/testsuite] Fix gdb.threads/fork-plus-threads.exp with check-readmore

[gdb/testsuite] Fix gdb.threads/fork-plus-threads.exp with check-readmore

When running test-case gdb.threads/fork-plus-threads.exp with check-readmore,
I run into:
...
[Inferior 11 (process 7029) exited normally]^M
[Inferior 1 (process 6956) exited normally]^M
FAIL: gdb.threads/fork-plus-threads.exp: detach-on-fork=off: \
  inferior 1 exited (timeout)
...

The problem is that the regexp consuming the "Inferior exited normally"
messages:
- consumes more than one of those messages at a time, but
- counts only one of those messages.

Fix this by adopting a line-by-line approach, which deals with those messages
one at a time.

Tested on x86_64-linux with native, check-read1 and check-readmore.