test: improve tests for custom field name logic

Author: codescape

src/test/java/de/codescape/jira/plugins/multiplesubtasks/model/CustomFieldsTest.java

@@ -5,6 +5,7 @@
 import static org.hamcrest.MatcherAssert.assertThat;
 import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.is;
+import static org.junit.Assert.fail;
 
 public class CustomFieldsTest {
 
@@ -32,4 +33,10 @@ public void extractCustomFieldNameForStringWithEscapedColonValue() {
         assertThat(result, is(equalTo("This:is:a:field")));
     }
 
+    @Test(expected = SyntaxFormatException.class)
+    public void extractCustomFieldNameForInvalidInput() {
+        CustomFields.extractCustomFieldName("CUSTOMFIELD(ILLEGAL)");
+        fail();
+    }
+
 }