filter-junit.go: fix loss of testcases when parsing Ginkgo v2 JUnit

The "junit" variable is meant to accumulate all testcases from the input
files. Overwriting it with the content of an input file when that input file
used the new Ginkgo V2 JUnit caused the result of prior input files to get
lost.

filter-junit.go

@@ -110,7 +110,7 @@ func main() {
 			if err := xml.Unmarshal(data, &junitv2); err != nil {
 				panic(err)
 			}
-			junit = junitv2.TestSuite
+			junit.TestCases = append(junit.TestCases, junitv2.TestSuite.TestCases...)
 		}
 	}