Add unit tests for Queue

src/test/groovy/io/github/aplotnikov/batch/processing/reactor/source/QueueSpec.groovy

@@ -0,0 +1,20 @@
+package io.github.aplotnikov.batch.processing.reactor.source
+
+import reactor.test.StepVerifier
+import spock.lang.Specification
+import spock.lang.Subject
+
+class QueueSpec extends Specification {
+
+    @Subject
+    Queue queue = new Queue(2, 0)
+
+    void 'should generate 2 files into Flux and Flux should be completed'() {
+        expect:
+            StepVerifier.create(queue.poll())
+                    .expectNext('client.xml')
+                    .expectNext('client.xml')
+                    .expectComplete()
+                    .verify()
+    }
+}