Skip attach-many-short-lived-threads.exp on known-broken DejaGnu versions

If the testsuite is run with a DejaGnu version that predates the fix
from last year:

  [PATCH] DejaGnu kills the wrong process due to PID-reuse races
  http://lists.gnu.org/archive/html/dejagnu/2015-07/msg00005.html

... gdb.threads/attach-many-short-lived-threads.exp fails randomly,
often.  Other tests randomly fail due to that issue too, but this one
is _much_ more exposed.

DejaGnu 1.6 was released meanwhile, which includes that DejaGnu fix,
and also some distros backported the fix too.

So skip the test when run with older/broken DejaGnus.

gdb/testsuite/ChangeLog:
2016-05-27  Pedro Alves  <palves@redhat.com>

	* gdb.threads/attach-many-short-lived-threads.exp (bad_dejagnu):
	New procedure.
	(top level): Call it, and bail out of DejaGnu is known to be bad.

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