stdlib: add dispatch tests for 'drop(while:)' and 'prefix(while:)'

Author: gribozavr

validation-test/stdlib/ExistentialCollection.swift.gyb

@@ -171,6 +171,14 @@ tests.test("${TestedType}: dispatch to wrapped") {
     _ = s.dropLast(0)
   }
 
+  Log.dropWhile.expectIncrement(Base.self) {
+    _ = s.drop { (_) in true }
+  }
+
+  Log.prefixWhile.expectIncrement(Base.self) {
+    _ = s.prefix { (_) in true }
+  }
+
   Log.prefixMaxLength.expectIncrement(Base.self) {
     _ = s.prefix(0)
   }