tests: add unit tests for ParselyTracker initialization

The `ParselyTracker#tearDown` method was added as singleton persisted between unit tests, causing some of them to fail. I could introduce a similar behavior using reflection, but I believe `internal tearDown` method is cleaner and is not problematic as not exposed to the client

parsely/src/main/java/com/parsely/parselyandroid/ParselyTracker.kt

@@ -16,7 +16,7 @@
 package com.parsely.parselyandroid
 
 import android.content.Context
-import kotlin.jvm.Throws
+import org.jetbrains.annotations.TestOnly
 
 /**
  * Tracks Parse.ly app views in Android apps
@@ -111,4 +111,9 @@ public object ParselyTracker {
         }
         instance = ParselyTrackerInternal(siteId, flushInterval, context, dryRun)
     }
+
+    @TestOnly
+    internal fun tearDown() {
+        instance = null
+    }
 }

parsely/src/test/java/com/parsely/parselyandroid/ParselyTrackerTest.kt

@@ -0,0 +1,40 @@
+package com.parsely.parselyandroid
+
+import org.junit.After
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.robolectric.RobolectricTestRunner
+import org.robolectric.RuntimeEnvironment
+
+@RunWith(RobolectricTestRunner::class)
+class ParselyTrackerTest {
+
+    @Test(expected = ParselyNotInitializedException::class)
+    fun `given no prior initialization, when executing a method, throw the exception`() {
+        ParselyTracker.engagementIsActive()
+    }
+
+    @Test(expected = ParselyAlreadyInitializedException::class)
+    fun `given prior initialization, when initializing, throw an exception`() {
+        ParselyTracker.init(siteId = "", context = RuntimeEnvironment.getApplication())
+
+        ParselyTracker.init(siteId = "", context = RuntimeEnvironment.getApplication())
+    }
+
+    @Test
+    fun `given no prior initialization, when initializing, do not throw any exception`() {
+        ParselyTracker.init(siteId = "", context = RuntimeEnvironment.getApplication())
+    }
+
+    @Test
+    fun `given tracker initialized, when calling a method, do not throw any exception`() {
+        ParselyTracker.init(siteId = "", context = RuntimeEnvironment.getApplication())
+
+        ParselyTracker.engagementIsActive()
+    }
+
+    @After
+    fun tearDown() {
+        ParselyTracker.tearDown()
+    }
+}