Fix racy test in gdb.base/new-ui.exp

Fix racy test in gdb.base/new-ui.exp

I noticed gdb.base/new-ui.exp failing once here with:

 FAIL: gdb.base/new-ui.exp: do_test: delete all breakpoints on extra console (got interactive prompt)
 FAIL: gdb.base/new-ui.exp: do_test: main console: next causes no spurious output on other console
 FAIL: gdb.base/new-ui.exp: do_test: main console: breakpoint hit reported on other console

The problem is 100% reproducible with check-read1:
  $ make check-read1 TESTS="gdb.*/new-ui.exp"

testsuite/gdb.log shows:
  delete
  Delete all breakpoints? (y or n) [answered Y; input not from terminal]
  (gdb) FAIL: gdb.base/new-ui.exp: do_test: delete all breakpoints on extra console (got interactive prompt)

This commit fixes the problem.

gdb/testsuite/ChangeLog:
2017-10-24  Pedro Alves  <palves@redhat.com>

	* gdb.base/new-ui.exp (do_test): Split "delete all breakpoints on
	extra console" test in two stages.