[gdb/testsuite] Fix gdb.dap/step-out.exp on s390x

[gdb/testsuite] Fix gdb.dap/step-out.exp on s390x

With test-case gdb.dap/step-out.exp on s390x-linux, I get:
...
>>> {"seq": 7, "type": "request", "command": "scopes", "arguments": {"frameId": 0}}
Content-Length: 569^M
^M
{"request_seq": 7, "type": "response", "command": "scopes", "body": {"scopes": [{"variablesReference": 1, "name": "Locals", "presentationHint": "locals", "expensive": false, "namedVariables": 1, "line": 35, "source": {"name": "step-out.c", "path": "/home/vries/gdb/src/gdb/testsuite/gdb.dap/step-out.c"}}, {"variablesReference": 2, "name": "Registers", "presentationHint": "registers", "expensive": false, "namedVariables": 114, "line": 35, "source": {"name": "step-out.c", "path": "/home/vries/gdb/src/gdb/testsuite/gdb.dap/step-out.c"}}]}, "success": true, "seq": 21}PASS: gdb.dap/step-out.exp: get scopes success
FAIL: gdb.dap/step-out.exp: three scopes
...

The problem is that the test-case expects three scopes:
...
lassign $scopes scope reg_scope return_scope
...
but the return_scope is missing because this doesn't work:
...
$ gdb -q -batch outputs/gdb.dap/step-out/step-out \
    -ex "b function_breakpoint_here" \
    -ex run \
    -ex finish
  ...
Value returned has type: struct result. Cannot determine contents
...

This is likely caused by a problem in gdb, but there's nothing wrong the DAP
support.

Fix this by:
- allowing two scopes, and
- declaring the tests of return_scope unsupported.

Tested on s390x-linux.

Approved-By: Tom Tromey <tom@tromey.com>