Make dependency on OpenTest4J optional again (fixes assertj#1563).

Author: PascalSchumacher

src/main/java/org/assertj/core/error/AssertionErrorCreator.java

@@ -109,7 +109,10 @@ private Optional<AssertionError> tryBuildingMultipleFailuresError(String heading
       if (multipleFailuresError instanceof AssertionError) { // means that we were able to build a MultipleFailuresError
         List<Throwable> failures = extractFailuresOf(multipleFailuresError);
         // we switch to AssertJMultipleFailuresError in order to control the formatting of the error message.
-        AssertionError assertionError = new AssertJMultipleFailuresError(heading, failures);
+        // we use reflection to avoid making opentest4j a required dependency
+        AssertionError assertionError = (AssertionError) constructorInvoker.newInstance("org.assertj.core.error.AssertJMultipleFailuresError",
+                                                                                        MULTIPLE_FAILURES_ERROR_ARGUMENT_TYPES,
+                                                                                        array(heading, failures));
         Failures.instance().removeAssertJRelatedElementsFromStackTraceIfNeeded(assertionError);
         return Optional.of(assertionError);
       }