Add one more test to cover LogbackTestHelper#resetLogback with fallback

This method is mocked in other tests, so is never exercised.
Test the delegation to LogbackTestHelpers by passing arguments
that don't exist, and validating the exception we get. Kind
of invasive but gets the job done without actually resetting
Logback, which we generally want to avoid.

src/test/java/org/kiwiproject/test/logback/LogbackTestHelperTest.java

@@ -1,5 +1,6 @@
 package org.kiwiproject.test.logback;
 
+import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException;
 import static org.mockito.ArgumentMatchers.anyString;
 import static org.mockito.Mockito.doNothing;
 import static org.mockito.Mockito.spy;
@@ -39,4 +40,15 @@ void shouldUseDefaultWhenCustomConfigurationIsBlank(String configFile) {
 
         verify(helper).resetLogback();
     }
+
+    @Test
+    void shouldDelegateToLogbackTestHelpersWithFallback() {
+        // Verify the delegation without actually resetting Logback
+        // by passing in config files that don't exist.
+
+        assertThatIllegalArgumentException()
+                .isThrownBy(() ->
+                        new LogbackTestHelper().resetLogback("acme-test-logback.xml", "acme-logback.xml"))
+                .withMessage("Did not find any of the Logback configurations: [acme-test-logback.xml, acme-logback.xml]");
+    }
 }