filter-junit.go: preserve system error log

Ginkgo v2 uses "system-err" to represent the log output of a test. This only
matters for failed tests, but for those it is important to preserve that
information.

filter-junit.go

@@ -56,6 +56,7 @@ type TestCase struct {
 	Name      string     `xml:"name,attr"`
 	Time      string     `xml:"time,attr"`
 	SystemOut string     `xml:"system-out,omitempty"`
+	SystemErr string     `xml:"system-err,omitempty"`
 	Failure   string     `xml:"failure,omitempty"`
 	Skipped   SkipReason `xml:"skipped,omitempty"`
 }