[gdb/testsuite] Fix interrupted sleep in multi-threaded test-cases

[gdb/testsuite] Fix interrupted sleep in multi-threaded test-cases

When running test-case gdb.threads/continue-pending-status.exp with native, I
have:
...
(gdb) continue^M
Continuing.^M
PASS: gdb.threads/continue-pending-status.exp: attempt 0: continue for ctrl-c
^C^M
Thread 1 "continue-pendin" received signal SIGINT, Interrupt.^M
[Switching to Thread 0x7ffff7fc4740 (LWP 1276)]^M
0x00007ffff758e4c0 in __GI___nanosleep () at nanosleep.c:27^M
27        return SYSCALL_CANCEL (nanosleep, requested_time, remaining);^M
(gdb) PASS: gdb.threads/continue-pending-status.exp: attempt 0: caught interrupt
...
but with target board unix/-m32, I run into:
...
(gdb) continue^M
Continuing.^M
PASS: gdb.threads/continue-pending-status.exp: attempt 0: continue for ctrl-c
[Thread 0xf74aeb40 (LWP 31957) exited]^M
[Thread 0xf7cafb40 (LWP 31956) exited]^M
[Inferior 1 (process 31952) exited normally]^M
(gdb) Quit^M
...

The problem is that the sleep (300) call at the end of main is interrupted,
which causes the inferior to exit before the ctrl-c can be send.

This problem is described at "Interrupted System Calls" in the docs, and the
suggested solution (using a sleep loop) indeed fixes the problem.

Fix this instead using the more prevalent:
...
  alarm (300);
  ...
  while (1) sleep (1);
...
which is roughly equivalent because the sleep is called at the end of main,
but slightly better because it guards against hangs from the start rather than
from the end of main.

Likewise in gdb.base/watch_thread_num.exp.

Likewise in gdb.btrace/enable-running.exp, but use the sleep loop there,
because the sleep is not called at the end of main.

Tested on x86_64-linux.