Generate concurrency-safe code in test bundle (#1274)

With the new concurrency features in Swift 6, there are also a set of
checks that ensure that the code is safe to run concurrently. Public
static mutable variables are considered unsafe unless they are
specifically of a type that conforms to `Sendable`. This patch changes
the test bundle generation to use `let` instead of `var` in the case of
`public static` properties.

Author: snprajwal

bin/make-test-bundle/Sources/make-test-bundle/Node/PropertyNode.swift

@@ -53,7 +53,7 @@ class PropertyNode: TypeMemberNode {
         if isDynamic {
             result += "\(levelString) \(kindString) var \(name.lowercased()): \(propertyType) { return \(propertyType)(\(propertyValue)) }\n"
         } else {
-            result += "\(levelString) \(kindString) var \(name.lowercased()): \(propertyType) = \(propertyValue)\n"
+            result += "\(levelString) \(kindString) \(kind == .static ? "let" : "var") \(name.lowercased()): \(propertyType) = \(propertyValue)\n"
         }
 
         if kind == .interface {