[Test] Use testRootDisposable instead of leaking test disposable in ApiTest

- `testDisposable` is already managed by `KtUsefulTestCase`, so disposal
  support comes for free. This replaces the previous `TestDisposable`,
  which was never disposed properly.

^KT-64099

js/js.tests/test/org/jetbrains/kotlin/js/testOld/api/ApiTest.kt

@@ -129,7 +129,7 @@ class ApiTest : KotlinTestWithEnvironment() {
     }
 
     override fun createEnvironment(): KotlinCoreEnvironment {
-        return KotlinCoreEnvironment.createForTests(TestDisposable(), CompilerConfiguration(), EnvironmentConfigFiles.JS_CONFIG_FILES)
+        return KotlinCoreEnvironment.createForTests(testRootDisposable, CompilerConfiguration(), EnvironmentConfigFiles.JS_CONFIG_FILES)
     }
 }