Test: convert empty method to borrowing getter

it's cheap enough that it can be a computed property; and gives more
test coverage.

(cherry picked from commit af9ce5f)

Author: kavon

test/Inputs/Swiftskell.swift

@@ -239,10 +239,12 @@ extension List where Element: ~Copyable {
 /// Basic utilities
 extension List where Element: ~Copyable {
   /// Is this list empty?
-  public borrowing func empty() -> Bool {
-    switch self {
-    case .empty: return true
-    case .cons(_, _): return false
+  public var isEmpty: Bool {
+    borrowing get {
+      switch self {
+      case .empty: true
+      case .cons(_, _): false
+      }
     }
   }
 

test/Interpreter/moveonly_swiftskell.swift

@@ -41,15 +41,15 @@ func testListBasic() {
   var items = List<File>(length: 5) { .init($0) }
   print(items.show())  // CHECK: [0, 1, 2, 3, 4, ]
   check(items.length() == 5)
-  check(!items.empty())
+  check(!items.isEmpty)
 
   items = items.reverse()
   check(items.length() == 5)
   print(items.show())  // CHECK: [4, 3, 2, 1, 0, ]
 
   items = .empty
   check(items.length() == 0)
-  check(items.empty())
+  check(items.isEmpty)
 
   let nums = List<Int>().push(7).push(7).push(3)
   print(nums.show()) // CHECK: [7, 7, 3, ]