[Wasm] add simple tests to track the size of wasm and mjs files

compiler/testData/codegen/box/size/add.kt

@@ -0,0 +1,15 @@
+// TARGET_BACKEND: WASM
+
+// WASM_DCE_EXPECTED_OUTPUT_SIZE: wasm 146_294 
+// WASM_DCE_EXPECTED_OUTPUT_SIZE:  mjs   8_247 
+
+// FILE: test.kt
+
+@JsExport
+fun add(a: Int, b: Int) = a + b
+
+// FILE: entry.mjs
+import k from "./index.mjs"
+
+const r = k.add(2, 3);
+if (r != 5) throw Error("Wrong result: " + r);