Fixes #2056. Make const_evaluation_A06_* tests weak (#2057)

Make const_evaluation_A06_* tests weak. Move the library to a `weak` directory.

Author: sgrekhov

LanguageFeatures/nnbd/const_evaluation_A06_t01.dart renamed to LanguageFeatures/nnbd/weak/const_evaluation_A06_t01.dart

@@ -9,10 +9,10 @@
 ///
 /// @author [email protected]
 
-// Requirements=nnbd-strong
+// Requirements=nnbd-weak
 
-import "const_evaluation_lib.dart";
-import "const_evaluation_legacy_lib.dart";
+import '../const_evaluation_lib.dart';
+import 'const_evaluation_legacy_lib.dart';
 
 const dynamic d = null;
 

LanguageFeatures/nnbd/const_evaluation_A06_t02.dart renamed to LanguageFeatures/nnbd/weak/const_evaluation_A06_t02.dart

@@ -8,11 +8,11 @@
 ///
 /// @author [email protected]
 
-// Requirements=nnbd-strong
+// Requirements=nnbd-weak
 
-import "../../Utils/expect.dart";
-import "const_evaluation_lib.dart";
-import "const_evaluation_legacy_lib.dart";
+import '../../../Utils/expect.dart';
+import '../const_evaluation_lib.dart';
+import 'const_evaluation_legacy_lib.dart';
 
 dynamic d = null;
 Never n = throw "Should not reach here";