create test to prove getService is not thread safe

Create test to confirm that ServiceBuilder.getService() should return
the same instance, even when accessed from two threads. To ensure
the test is consistent we made the constructor slow so the thread both
end on the same line of code.

Author: jkasten2

OneSignalSDK/onesignal/core/src/test/java/com/onesignal/common/ServiceProviderTest.kt

@@ -0,0 +1,44 @@
+package com.onesignal.common
+
+import com.onesignal.common.services.ServiceBuilder
+import com.onesignal.common.services.ServiceProvider
+import io.kotest.core.spec.style.FunSpec
+import io.kotest.matchers.types.shouldBeSameInstanceAs
+import java.util.concurrent.LinkedBlockingQueue
+
+internal interface IMyTestInterface
+
+internal class MySlowConstructorClass : IMyTestInterface {
+    init {
+        // NOTE: Keep these println calls, otherwise Kotlin optimizes
+        //   something which cases the test not fail when it should.
+        println("MySlowConstructorClass BEFORE")
+        Thread.sleep(10)
+        println("MySlowConstructorClass AFTER")
+    }
+}
+
+class ServiceProviderTest : FunSpec({
+
+    fun setupServiceProviderWithSlowInitClass(): ServiceProvider {
+        val serviceBuilder = ServiceBuilder()
+        serviceBuilder.register<MySlowConstructorClass>().provides<IMyTestInterface>()
+        return serviceBuilder.build()
+    }
+
+    test("getService is thread safe") {
+        val services = setupServiceProviderWithSlowInitClass()
+
+        val queue = LinkedBlockingQueue<IMyTestInterface>()
+        Thread {
+            queue.add(services.getService<IMyTestInterface>())
+        }.start()
+        Thread {
+            queue.add(services.getService<IMyTestInterface>())
+        }.start()
+
+        val firstReference = queue.take()
+        val secondReference = queue.take()
+        firstReference shouldBeSameInstanceAs secondReference
+    }
+})