Test step-over-{lands-on-breakpoint|trips-on-watchpoint}.exp with displaced stepping

Test step-over-{lands-on-breakpoint|trips-on-watchpoint}.exp with displaced stepping

These tests exercise the infrun.c:proceed code that needs to know to
start new step overs (along with switch_back_to_stepped_thread, etc.).
That code is tricky to get right in the multitude of possible
combinations (at least):

 (native | remote)
  X (all-stop | all-stop-but-target-always-in-non-stop)
  X (displaced-stepping | in-line step-over).

The first two above are properties of the target, but the different
step-over-breakpoint methods should work with any target that supports
them.  This patch makes sure we always test both methods on all
targets.

Tested on x86-64 Fedora 20.

gdb/testsuite/ChangeLog:
2015-04-10  Pedro Alves  <palves@redhat.com>

	* gdb.threads/step-over-lands-on-breakpoint.exp (do_test): New
	procedure, factored out from ...
	(top level): ... here.  Add "set displaced-stepping" testing axis.
	* gdb.threads/step-over-trips-on-watchpoint.exp (do_test): New
	parameter "displaced".  Use it.
	(top level): Use foreach and add "set displaced-stepping" testing
	axis.