[tests] Fix and move erroneous constructor tearoff dot shorthands test.

Pretty sure this isn't valid code. Moving to an error test.

Bug: https://github.com/dart-lang/sdk/issues/59758
Change-Id: I58fc9cc00286bfae2b6093cfb5ac2c215baf344d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/414187
Commit-Queue: Kallen Tu <kallentu@google.com>
Reviewed-by: Lasse Nielsen <lrn@google.com>

tests/language/dot_shorthands/constructor/constructor_error_test.dart

@@ -42,4 +42,9 @@ void main() {
   // ^
   // [analyzer] unspecified
   // [cfe] unspecified
+
+  UnnamedConstructor Function() ctorTearoff = .new;
+  // ^
+  // [analyzer] unspecified
+  // [cfe] unspecified
 }

tests/language/dot_shorthands/constructor/constructor_test.dart

@@ -57,8 +57,6 @@ void main() {
   ConstructorExt? ctorExtNullable5 = .constNamed(x: x);
   ConstructorExt? ctorExtNullable6 = .constOptional(x);
 
-  UnnamedConstructor Function() ctorTearoff = .new;
-
   // Parameter context type.
   ConstructorClassContext(.new(1));
   ConstructorClassContext.named(x: .optional(1));