[gdb/testsuite] Fix gdb.threads/manythreads.exp with check-read1

[gdb/testsuite] Fix gdb.threads/manythreads.exp with check-read1

When running test-case gdb.threads/manythreads.exp with check-read1, I ran
into this hard-to-reproduce FAIL:
...
[New Thread 0x7ffff7318700 (LWP 31125)]^M
[Thread 0x7ffff7321700 (LWP 31124) exited]^M
[New T^C^M
^M
Thread 769 "manythreads" received signal SIGINT, Interrupt.^M
[Switching to Thread 0x7ffff6d66700 (LWP 31287)]^M
0x00007ffff7586a81 in clone () from /lib64/libc.so.6^M
(gdb) FAIL: gdb.threads/manythreads.exp: stop threads 1
...

The matching in the failing gdb_test_multiple is done in an intricate way,
trying to pass on some order and fail on another order.

Fix this by rewriting the regexps to match one line at most, and detecting
invalid order by setting and checking state variables.

Tested on x86_64-linux.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29177