script: move testbinding DOM interfaces to script/dom/test (#39585)

Move testbinding interfaces to it's own module, to avoid cluttering dom
folder.

Testing: A successful build is enough
Part of #38901

---------

Signed-off-by: Gae24 <96017547+Gae24@users.noreply.github.com>

components/script/dom/testing/testbindingproxy.rs

@@ -0,0 +1,46 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+// check-tidy: no specs after this line
+
+use dom_struct::dom_struct;
+
+use crate::dom::bindings::codegen::Bindings::TestBindingProxyBinding::TestBindingProxyMethods;
+use crate::dom::bindings::str::DOMString;
+use crate::dom::testbinding::TestBinding;
+
+#[dom_struct]
+pub(crate) struct TestBindingProxy {
+    testbinding_: TestBinding,
+}
+
+impl TestBindingProxyMethods<crate::DomTypeHolder> for TestBindingProxy {
+    fn Length(&self) -> u32 {
+        0
+    }
+    fn SupportedPropertyNames(&self) -> Vec<DOMString> {
+        vec![]
+    }
+    fn GetNamedItem(&self, _: DOMString) -> DOMString {
+        DOMString::new()
+    }
+    fn SetNamedItem(&self, _: DOMString, _: DOMString) {}
+    fn GetItem(&self, _: u32) -> DOMString {
+        DOMString::new()
+    }
+    fn SetItem(&self, _: u32, _: DOMString) {}
+    fn RemoveItem(&self, _: DOMString) {}
+    fn Stringifier(&self) -> DOMString {
+        DOMString::new()
+    }
+    fn IndexedGetter(&self, _: u32) -> Option<DOMString> {
+        None
+    }
+    fn NamedDeleter(&self, _: DOMString) {}
+    fn IndexedSetter(&self, _: u32, _: DOMString) {}
+    fn NamedSetter(&self, _: DOMString, _: DOMString) {}
+    fn NamedGetter(&self, _: DOMString) -> Option<DOMString> {
+        None
+    }
+}