Make gdb.base/find-unmapped.exp pass on remote targets

Currently, with --target_board=native-extended-gdbserver, we get:

  Running .../src/gdb/testsuite/gdb.base/find-unmapped.exp ...
  FAIL: gdb.base/find-unmapped.exp: find global_var_0, global_var_2, 0xff
  FAIL: gdb.base/find-unmapped.exp: find global_var_1, global_var_2, 0xff
  FAIL: gdb.base/find-unmapped.exp: find global_var_2, (global_var_2 + 16), 0xff

This commit makes the test pass there, and also enables in on
--target_board=native-gdbserver, and other remote targets.

I've filed PR gdb/22293 to track the missing-warning problem.

gdb/testsuite/ChangeLog:
2017-10-13  Pedro Alves  <palves@redhat.com>

	PR gdb/22293
	* gdb.base/find-unmapped.exp: Don't skip if is_remote target.
	(top level): Move some tests to ...
	(test_not_found): ... this new procedure.
	(top level): Call it.