enable by default

Author: thomas-k-cameron

codegen-core/src/main/kotlin/software/amazon/smithy/rust/codegen/core/testutil/Rust.kt

@@ -347,7 +347,7 @@ fun FileManifest.printGeneratedFiles() {
 fun TestWriterDelegator.compileAndTest(
     runClippy: Boolean = false,
     expectFailure: Boolean = false,
-    enableUnstableFlag: Boolean = false,
+    enableUnstableFlag: Boolean = true,
 ): String {
     val stubModel = """
         namespace fake