[enhancement] export error QName to allow fallback

Allow backwards compatible implementations of custom assertions
for packages that target multiple version of eXist-db, including
ones that do not yet have `test:fail`.

Author: line-o

exist-core/src/main/resources/org/exist/xquery/lib/xqsuite/xqsuite.xql

@@ -58,6 +58,7 @@ declare variable $test:UNKNOWN_ASSERTION := QName($test:TEST_NAMESPACE, "no-such
 declare variable $test:WRONG_ARG_COUNT := QName($test:TEST_NAMESPACE, "wrong-number-of-arguments");
 declare variable $test:TYPE_ERROR := QName($test:TEST_NAMESPACE, "type-error");
 declare variable $test:UNKNOWN_ANNOTATION_VALUE_TYPE := QName($test:TEST_NAMESPACE, "unknown-annotation-value-type");
+declare variable $test:FAILURE := QName($test:TEST_NAMESPACE, "failure");
 declare variable $test:CUSTOM_ASSERTION_FAILURE_TYPE := "custom-assertion-failure";
 
 (:~
@@ -150,7 +151,7 @@ declare function test:fail (
     $actual as item()*,
     $type as xs:string
 ) as empty-sequence() {
-    error(xs:QName("test:failure"), $message, map {
+    error($test:FAILURE, $message, map {
         "expected": $expected,
         "actual": $actual,
         "type": $type

exist-core/src/test/xquery/xqsuite/custom-assertion.xqm

@@ -53,6 +53,21 @@ function ca:test-fail-4() as item()* {
     }
 };
 
+declare
+    %test:assertEquals("Custom message", "expected", "actual", "custom-assertion-failure")
+function ca:test-fail-fallback() as item()* {
+    try {
+        error($test:FAILURE, "Custom message", map {
+            "expected": "expected",
+            "actual": "actual",
+            "type": $test:CUSTOM_ASSERTION_FAILURE_TYPE
+        })
+    }
+    catch test:failure {
+        $err:description, $err:value?expected, $err:value?actual, $err:value?type
+    }
+};
+
 declare
     %test:assertTrue
 function ca:map-assertion-pass() as item()* {