[gdb/testsuite] Fix scopes check in gdb.dap/rust-slices.exp

When running test-case gdb.dap/rust-slices.exp on aarch64-linux
(debian 12/bookworm), I run into:
...
{"request_seq": 6, "type": "response", "command": "scopes", "body": {"scopes": [{"variablesReference": 1, "name": "Locals", "presentationHint": "locals", "expensive": false, "namedVariables": 3, "line": 28, "source": {"name": "rust-slices.rs", "path": "/home/linux/gdb/binutils-gdb.git/gdb/testsuite/gdb.dap/rust-slices.rs"}}, {"variablesReference": 2, "name": "Registers", "presentationHint": "registers", "expensive": false, "namedVariables": 261, "line": 28, "source": {"name": "rust-slices.rs", "path": "/home/linux/gdb/binutils-gdb.git/gdb/testsuite/gdb.dap/rust-slices.rs"}}]}, "success": true, "seq": 20}PASS: gdb.dap/rust-slices.exp: get scopes success
FAIL: gdb.dap/rust-slices.exp: three scopes
PASS: gdb.dap/rust-slices.exp: scope is locals
PASS: gdb.dap/rust-slices.exp: locals presentation hint
PASS: gdb.dap/rust-slices.exp: three vars in scope
...

The test-case expects three scopes due to a rust compiler issue:
...
 # There are three scopes because an artificial symbol ends up in the
 # DWARF.  See https://github.com/rust-lang/rust/issues/125126

.
 gdb_assert {[llength $scopes] == 3} "three scopes"
...
but it seems that the version used here (rustc 1.63.0, llvm 14.0.6) doesn't
have this issue.

Fix this by allowing two or three scopes, and changing the test name to
"two scopes".

Tested on aarch64-linux.

Approved-by: Kevin Buettner <kevinb@redhat.com>

PR testsuite/31983
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31983