Move ScopeSubject from test/ to src/

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=237314967

test/com/google/javascript/jscomp/ScopeSubject.java → src/com/google/javascript/jscomp/testing/ScopeSubject.java

@@ -84,6 +84,7 @@ public DeclarationSubject declares(String name) {
     return actual().hasSlot(name) ? checkNotNull(actual().getVar(name)) : null;
   }
 
+  /** A subject for an {@link AbstractVar} declared by this particular scope. */
   public final class DeclarationSubject {
     private final AbstractVar<?, ?> var;