Add tests and simplify existing ones

Author: aozgaa

tests/cases/fourslash/codeFixChangeExtendsToImplements.ts

@@ -0,0 +1,6 @@
+/// <reference path='fourslash.ts' />
+
+//// interface I {}
+//// [|class C extends I|]{}
+
+verify.rangeAfterCodeFix("class C implements I");

tests/cases/fourslash/codeFixChangeExtendsToImplementsFS1.ts

@@ -0,0 +1,6 @@
+/// <reference path='fourslash.ts' />
+
+////interface I<X> { x: X}
+////[|class C<T extends string , U> extends I<T>|]{}
+
+verify.rangeAfterCodeFix("class C<T extends string , U> implements I<T>");