Add new test for box syntax

Add a new test to prevent regressions on the box syntax as well as its
feature gate.

gcc/testsuite/ChangeLog:

	* rust/compile/box_syntax.rs: New test.
	* rust/compile/box_syntax_feature_gate.rs: New test.

Signed-off-by: Pierre-Emmanuel Patry <[email protected]>

gcc/testsuite/rust/compile/box_syntax.rs

@@ -0,0 +1,6 @@
+// { dg-options "-fsyntax-only" }
+#![feature(box_syntax)]
+
+fn main() {
+    let x: Box<_> = box 1;
+}

gcc/testsuite/rust/compile/box_syntax_feature_gate.rs

@@ -0,0 +1,5 @@
+// { dg-options "-frust-compile-until=lowering" }
+
+fn main() {
+    let x: Box<_> = box 1; //{ dg-error "box expression syntax is experimental." "" { target *-*-* }  }
+}