Add test to verify #26669 where declaration output is incorrect when declaration flag is not set explicitly in options

tests/cases/compiler/declarationEmitWithComposite.ts

@@ -0,0 +1,13 @@
+// @composite: true
+// @fullEmitPaths: true
+
+// @filename: /foo/tsconfig.json
+{
+    "compilerOptions": { "composite": true, "outDir": "out" }
+}
+
+// @filename: /foo/test.ts
+interface Foo {
+    x: number;
+}
+export default Foo;