Add espresso test for usernames.

app/src/androidTest/java/org/thoughtcrime/securesms/testing/RxTestSchedulerRule.kt

@@ -0,0 +1,36 @@
+package org.thoughtcrime.securesms.testing
+
+import io.reactivex.rxjava3.plugins.RxJavaPlugins
+import io.reactivex.rxjava3.schedulers.TestScheduler
+import org.junit.rules.ExternalResource
+
+/**
+ * JUnit Rule which initialises Rx thread schedulers. If a specific
+ * scheduler is not specified, it defaults to the `defaultTestScheduler`
+ */
+class RxTestSchedulerRule(
+  val defaultTestScheduler: TestScheduler = TestScheduler(),
+  val ioTestScheduler: TestScheduler = defaultTestScheduler,
+  val computationTestScheduler: TestScheduler = defaultTestScheduler,
+  val singleTestScheduler: TestScheduler = defaultTestScheduler,
+  val newThreadTestScheduler: TestScheduler = defaultTestScheduler,
+) : ExternalResource() {
+
+  override fun before() {
+    RxJavaPlugins.setInitIoSchedulerHandler { ioTestScheduler }
+    RxJavaPlugins.setIoSchedulerHandler { ioTestScheduler }
+
+    RxJavaPlugins.setInitComputationSchedulerHandler { computationTestScheduler }
+    RxJavaPlugins.setComputationSchedulerHandler { computationTestScheduler }
+
+    RxJavaPlugins.setInitSingleSchedulerHandler { singleTestScheduler }
+    RxJavaPlugins.setSingleSchedulerHandler { singleTestScheduler }
+
+    RxJavaPlugins.setInitNewThreadSchedulerHandler { newThreadTestScheduler }
+    RxJavaPlugins.setNewThreadSchedulerHandler { newThreadTestScheduler }
+  }
+
+  override fun after() {
+    RxJavaPlugins.reset()
+  }
+}