rustdoc-json: Add test for #[optimize(..)]

Follow up to https://www.github.com/rust-lang/rust/pull/138291

Author: aDotInTheVoid

tests/rustdoc-json/attrs/optimize.rs

@@ -0,0 +1,13 @@
+#![feature(optimize_attribute)]
+
+//@ is "$.index[?(@.name=='speed')].attrs" '["#[attr = Optimize(Speed)]"]'
+#[optimize(speed)]
+pub fn speed() {}
+
+//@ is "$.index[?(@.name=='size')].attrs" '["#[attr = Optimize(Size)]"]'
+#[optimize(size)]
+pub fn size() {}
+
+//@ is "$.index[?(@.name=='none')].attrs" '["#[attr = Optimize(DoNotOptimize)]"]'
+#[optimize(none)]
+pub fn none() {}