JS: remove MULTIPLATFORM directive from box tests

Move all sources into one module and use "!LANGUAGE:
+MultiPlatformProjects" instead

js/js.translator/testData/box/kotlin.test/mpp.kt

@@ -1,7 +1,6 @@
+// !LANGUAGE: +MultiPlatformProjects
 // EXPECTED_REACHABLE_NODES: 1173
-// MULTIPLATFORM
 
-// MODULE: lib
 // FILE: lib.kt
 import kotlin.test.Test
 
@@ -9,7 +8,6 @@ expect class PlatformTest {
     @Test fun platformTest()
 }
 
-// MODULE: main(lib)
 // FILE: main.kt
 import kotlin.test.Test
 
@@ -24,4 +22,4 @@ fun box() = checkLog {
         test("platformTest")
         test("someOtherTest")
     }
-}
+}