[Wasm] Don't write .wat files in box tests

These files take space on CI, but they are not validated or processed
automatically in any way.

Disabled by default, enabled via kotlin.wasm.debugMode=1

compiler/ir/backend.wasm/src/org/jetbrains/kotlin/backend/wasm/compiler.kt

@@ -168,9 +168,12 @@ fun writeCompilationResult(
     dir: File,
     loaderKind: WasmLoaderKind,
     fileNameBase: String = "index",
+    writeWat: Boolean = true,
 ) {
     dir.mkdirs()
-    File(dir, "$fileNameBase.wat").writeText(result.wat)
+    if (writeWat) {
+        File(dir, "$fileNameBase.wat").writeText(result.wat)
+    }
     File(dir, "$fileNameBase.wasm").writeBytes(result.wasm)
     val jsWithLoader = generateJsWasmLoader(loaderKind, "./$fileNameBase.wasm", result.js)
     File(dir, "$fileNameBase.js").writeText(jsWithLoader)