Add testcase for GDB hang fixed by previous commit

Add testcase for GDB hang fixed by previous commit

This adds a testcase for the previous commit.  The regression was
related to in-line step overs.  The reason we didn't see it on native
x86-64/s390 GNU/Linux testing is that native debugging uses displaced
stepping by default (because native debugging defaults to "maint set
target-non-stop on"), unlike remote debugging.

So in order to trigger the bug with native debugging as well, the
testcase disables displaced stepping explicitly.

Also, instead of using watchpoints to trigger the regression, the
testcase uses a breakpoint at address 0, which should be more
portable.

gdb/testsuite/ChangeLog:
2018-01-12  Pedro Alves  <palves@redhat.com>

	* gdb.base/continue-after-aborted-step-over.c: New.
	* gdb.base/continue-after-aborted-step-over.exp: New.