OZG-4095 receive: Use test string constants

src/test/java/de/ozgcloud/nachrichten/postfach/osiv2/transfer/Osi2ResponseMapperTest.java

@@ -34,7 +34,7 @@ class Osi2ResponseMapperTest {
 		void shouldMapVorgangId() {
 			var result = doMapping();
 
-			assertThat(result.getVorgangId()).isEqualTo("OZG-Cloud-VorgangId");
+			assertThat(result.getVorgangId()).isEqualTo(SEQUENCE_NUMMER);
 		}
 
 		@Test
@@ -63,7 +63,7 @@ class Osi2ResponseMapperTest {
 		void shouldMapSubject() {
 			var result = doMapping();
 
-			assertThat(result.getSubject()).isEqualTo("Das ist das Subject");
+			assertThat(result.getSubject()).isEqualTo(SUBJECT);
 		}
 
 		@Test