Add method to declare expected unsubscribed completable counts in tests

We have tests where `Completable` instances get created, but not
subscribed to and this is intentional (for instance, verifying that
certain things do *NOT* happen). For these cases, we need some way to
indicate to the test rule that we expect a certain test to not subscribe
to anything.

This commit adds support for this by adding a meta annotation,
`ExpectUnsubscribed`, which can be used to annotate a test method.

Author: Vinay S Shenoy

app/src/sharedTest/java/org/simple/clinic/util/ExpectUnsubscribed.kt

@@ -0,0 +1,5 @@
+package org.simple.clinic.util
+
+@Retention(AnnotationRetention.RUNTIME)
+@Target(AnnotationTarget.FUNCTION)
+annotation class ExpectUnsubscribed(val completables: Int = 0)

app/src/sharedTest/java/org/simple/clinic/util/RxErrorsRule.kt

@@ -40,7 +40,7 @@ class RxErrorsRule : TestRule {
           subscriptionTracker.stopTracking()
 
           assertNoErrors()
-          subscriptionTracker.assertAllCompletablesSubscribed()
+          subscriptionTracker.assertAllCompletablesSubscribed(description.getAnnotation(ExpectUnsubscribed::class.java))
         }
       }
     }

app/src/sharedTest/java/org/simple/clinic/util/RxJavaSubscriptionTracker.kt

@@ -23,13 +23,16 @@ class RxJavaSubscriptionTracker {
     RxJavaPlugins.setOnCompletableAssembly(null)
   }
 
-  fun assertAllCompletablesSubscribed() {
+  fun assertAllCompletablesSubscribed(
+      expectUnsubscribed: ExpectUnsubscribed? = null
+  ) {
     val assembledCount = assembledCompletables.size
     val subscribedCount = assembledCompletables.count { it.hasBeenSubscribedTo }
-    val haveAllBeenSubscribed = assembledCount == subscribedCount
+    val expectedUnsubscribedCount = expectUnsubscribed?.completables ?: 0
+    val totalUnsubscribedCount = assembledCount - subscribedCount
 
-    if (!haveAllBeenSubscribed) {
-      throw AssertionError("Assembled $assembledCount Completables, but only subscribed to $subscribedCount!")
+    if (totalUnsubscribedCount != expectedUnsubscribedCount) {
+      throw AssertionError("Assembled $assembledCount Completables, but only subscribed to $subscribedCount, expecting $expectedUnsubscribedCount to not be subscribed")
     }
   }