Simplify midnight timer interface for testing

uhabits-core/src/jvmMain/java/org/isoron/uhabits/core/utils/MidnightTimer.kt

@@ -57,7 +57,7 @@ open class MidnightTimer @Inject constructor() {
         }
     }
 
-    interface MidnightListener {
+    fun interface MidnightListener {
         fun atMidnight()
     }
 }

uhabits-core/src/jvmTest/java/org/isoron/uhabits/core/utils/MidnightTimerTest.kt

@@ -25,14 +25,8 @@ class MidnightTimerTest : BaseUnitTest() {
             DateUtils.setFixedLocalTime(unixTime(2017, Calendar.JANUARY, 1, 23, 59, DateUtils.MINUTE_LENGTH - 1))
 
             val suspendedListener = suspendCoroutine<Boolean> { continuation ->
-                val listener = object : MidnightTimer.MidnightListener {
-                    override fun atMidnight() {
-                        continuation.resume(true)
-                    }
-                }
-
                 MidnightTimer().apply {
-                    addListener(listener)
+                    addListener { continuation.resume(true) }
                     // When
                     onResume(1, executor)
                 }