[gdb/testsuite] Handle output after prompt in gdb.threads/step-N-all-progress.exp

[gdb/testsuite] Handle output after prompt in gdb.threads/step-N-all-progress.exp

Using "taskset -c 0" I run into this timeout:
...
(gdb) PASS: gdb.threads/step-N-all-progress.exp: non-stop=on: \
  target-non-stop=on: continue to breakpoint: break here
next 3^M
[New Thread 0x7ffff7dbd6c0 (LWP 10202)]^M
50        return 0;^M
(gdb) [Thread 0x7ffff7dbd6c0 (LWP 10202) exited]^M
FAIL: gdb.threads/step-N-all-progress.exp: non-stop=on: target-non-stop=on: \
  next 3 (timeout)
...

The problem is that this test:
...
    gdb_test "next 3" "return 0;"
...
expects no output after the prompt.

Fix this by using -no-prompt-anchor.

Tested on x86_64-linux.