TestScheduler not working while testing specific usecase

I am testing single use-case. but test observer will not emit any thing in test environment.

• even if i change subscribeOn() from Schedulers.newThread() to TestScheduler() still test fail. i am confuse. and i don't know what i am doing wrong here?
• if i remove subscribeOn,observeOn method then test pass successfully.then what is the actual use of TestScheduler() ?

following are the UseCase.kt file

package com.*.domain.interactors

import com.*.domain.executors.PostExecutionThread
import io.reactivex.Observable
import io.reactivex.schedulers.Schedulers

/**
 * *
 *
 * Created by mcd on 3/11/2018.
 */
abstract class UseCase<T>(val postExecutionThread: PostExecutionThread) {

    abstract fun buildUseCaseBuilder(): Observable<T>

    /**
     * execute method for observables
     */
    open fun execute(): Observable<T> {
        return buildUseCaseBuilder()
                .subscribeOn(Schedulers.newThread())
                .observeOn(postExecutionThread.getScheduler())
    }
}

following the test class with above class under test

package com.*.domain

import com.*.domain.executors.PostExecutionThread
import com.*.domain.interactors.UseCase
import io.reactivex.Observable
import io.reactivex.Observer
import io.reactivex.disposables.Disposable
import io.reactivex.observers.TestObserver
import io.reactivex.schedulers.TestScheduler
import org.junit.Before
import org.junit.Test
import org.mockito.Mockito
import org.mockito.Mockito.`when` as _when

@Suppress("RemoveRedundantBackticks")
/**
 * *
 *
 * Created by mcd on 3/19/2018.
 */
class UseCaseTest {

    val postExecutionThread=Mockito.mock(PostExecutionThread::class.java)
    val result: String="test"
    val testObserver=TestObserver<String>()

    @Before
    fun setUp() {
        _when(postExecutionThread.getScheduler()).thenReturn(TestScheduler())
    }

    @Test
    fun `test execute method of use case`() {
        println(" thread type ${postExecutionThread.getScheduler()}")

        //test fail
        //java.lang.AssertionError: Value count differs; Expected: 1 [test], Actual: 0 [] (latch = 1, values = 0, errors = 0, completions = 0)
        TestUseCase(postExecutionThread).execute().test()
                .assertResult(result)

        //subscriber not print any thing
        TestUseCase(postExecutionThread).execute().subscribe{
            println("called $it")
        }
    }

    inner class TestUseCase(postExecutionThread: PostExecutionThread?) : UseCase<String>(postExecutionThread!!) {
        override fun buildUseCaseBuilder(): Observable<String> {
            return Observable.just(result)
        }
    }
}