[gdb/testsuite] Fix gdb.threads/watchpoint-fork.exp race

[gdb/testsuite] Fix gdb.threads/watchpoint-fork.exp race

I ran into:
...
Thread 3.1 "watchpoint-fork" hit Breakpoint 3, marker () at \
  watchpoint-fork-mt.c:42^M
42      }^M
(gdb) parent2: 1945^M

FAIL: gdb.threads/watchpoint-fork.exp: child: multithreaded: breakpoint (A) \
  after the second fork (timeout)
...
The problem is that the FAILing gdb_test expects '(gdb) ' to be the last thing
printed, but the inferior prints something after that.

A similar FAIL is described in the sources in watchpoint-fork-parent.c:
...
      printf ("child%d: %d\n", nr, (int) getpid ());
      /* Delay to get both the "child%d" and "parent%d" message printed
      	 without a race breaking expect by its endless wait on `$gdb_prompt$':
         Breakpoint 3, marker () at watchpoint-fork.c:33
         33      }
         (gdb) parent2: 14223  */
      i = sleep (1);
...

I noticed that while the executables print output, the output is not verified in
the test-case, so it's merely debug output.

Fix this by:
- guarding the prints in the executables (as well as related
  sleep and setbuf calls) with #if DEBUG, and
- compiling by default with DEBUG=0.

gdb/testsuite/ChangeLog:

2020-01-29  Tom de Vries  <tdevries@suse.de>

	* gdb.threads/watchpoint-fork-child.c: Guard prints with #if DEBUG.
	* gdb.threads/watchpoint-fork-mt.c: Same.
	* gdb.threads/watchpoint-fork-parent.c: Same.
	* gdb.threads/watchpoint-fork-st.c: Same.
	* gdb.threads/watchpoint-fork.exp: Compile with DEBUG=0.

Change-Id: I63efd4c7771f96b5f5cd87ef2ab36795484ae2be