Move tests for builder inference into the corresponding directory

compiler/testData/codegen/box/inference/builderInference/callableReferenceAndCoercionToUnit.kt

@@ -0,0 +1,20 @@
+// DONT_TARGET_EXACT_BACKEND: WASM
+// WASM_MUTE_REASON: STDLIB_COLLECTIONS
+// !LANGUAGE: +NewInference
+// !DIAGNOSTICS: -EXPERIMENTAL_API_USAGE_ERROR -UNUSED_EXPRESSION
+// WITH_RUNTIME
+
+@OptIn(ExperimentalStdlibApi::class)
+fun test(s: String?): Int {
+    val list = buildList {
+        s?.let(::add)
+    }
+    return list.size
+}
+
+fun box(): String {
+    return when (test("hello")) {
+        1 -> "OK"
+        else -> "Error"
+    }
+}