CORDA-649: Second attempt to make PersistentNetworkMapCacheTest more stable (#1738)

testing/test-utils/src/main/kotlin/net/corda/testing/Eventually.kt

@@ -0,0 +1,24 @@
+package net.corda.testing
+
+import java.time.Duration
+
+/**
+ * Ideas borrowed from "io.kotlintest" with some improvements made
+ * This is meant for use from Kotlin code use only mainly due to it's inline/reified nature
+ */
+inline fun <reified E : Throwable, R>eventually(duration: Duration, f: () -> R): R {
+    val end = System.nanoTime() + duration.toNanos()
+    var times = 0
+    while (System.nanoTime() < end) {
+        try {
+            return f()
+        } catch (e: Throwable) {
+            when(e) {
+                is E -> {}// ignore and continue
+                else -> throw e // unexpected exception type - rethrow
+            }
+        }
+        times++
+    }
+    throw AssertionError("Test failed after $duration; attempted $times times")
+}