Move common go-junit-report code into separate internal package

There was some code duplication between the go-junit-report binary, its
tests and the testdata/generate-golden script. This has been moved into
an internal package. The go-junit-report binary can now just focus on
flag parsing and validation, and it should be less likely that the
binary, tests and golden report generator behave differently.

internal/gojunitreport/go-junit-report_test.go

@@ -0,0 +1,107 @@
+package gojunitreport
+
+import (
+	"bytes"
+	"flag"
+	"fmt"
+	"os"
+	"path/filepath"
+	"regexp"
+	"strconv"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/google/go-cmp/cmp"
+)
+
+const testDataDir = "../../testdata/"
+
+var matchTest = flag.String("match", "", "only test testdata matching this pattern")
+
+var testConfigs = map[int]Config{
+	5: {SkipXMLHeader: true},
+	6: {SkipXMLHeader: true},
+	7: {PackageName: "test/package"},
+}
+
+func TestRun(t *testing.T) {
+	matchRegex := compileMatch(t)
+
+	files, err := filepath.Glob(testDataDir + "*.txt")
+	if err != nil {
+		t.Fatalf("error finding files in testdata: %v", err)
+	}
+
+	for _, file := range files {
+		if !matchRegex.MatchString(file) {
+			continue
+		}
+
+		conf, reportFile, err := testFileConfig(strings.TrimPrefix(file, testDataDir))
+		if err != nil {
+			t.Errorf("testFileConfig error: %v", err)
+			continue
+		}
+
+		t.Run(file, func(t *testing.T) {
+			testRun(file, testDataDir+reportFile, conf, t)
+		})
+	}
+}
+
+func testRun(inputFile, reportFile string, config Config, t *testing.T) {
+	input, err := os.Open(inputFile)
+	if err != nil {
+		t.Fatalf("error opening input file: %v", err)
+	}
+	defer input.Close()
+
+	wantReport, err := os.ReadFile(reportFile)
+	if os.IsNotExist(err) {
+		t.Skipf("Skipping test with missing report file: %s", reportFile)
+	} else if err != nil {
+		t.Fatalf("error loading report file: %v", err)
+	}
+
+	config.Parser = "gotest"
+	if strings.HasSuffix(inputFile, ".gojson.txt") {
+		config.Parser = "gojson"
+	}
+	config.Hostname = "hostname"
+	config.Properties = map[string]string{"go.version": "1.0"}
+	config.TimestampFunc = func() time.Time {
+		return time.Date(2022, 1, 1, 0, 0, 0, 0, time.UTC)
+	}
+
+	var output bytes.Buffer
+	if _, err := config.Run(input, &output); err != nil {
+		t.Fatal(err)
+	}
+
+	if diff := cmp.Diff(output.String(), string(wantReport)); diff != "" {
+		t.Errorf("Unexpected report diff (-want, +got):\n%v", diff)
+	}
+}
+
+func testFileConfig(filename string) (config Config, reportFile string, err error) {
+	var prefix string
+	if idx := strings.IndexByte(filename, '-'); idx < 0 {
+		return config, "", fmt.Errorf("testdata file does not contain a dash (-); expected name `{id}-{name}.txt` got `%s`", filename)
+	} else {
+		prefix = filename[:idx]
+	}
+	id, err := strconv.Atoi(prefix)
+	if err != nil {
+		return config, "", fmt.Errorf("testdata file did not start with a valid number: %w", err)
+	}
+	return testConfigs[id], fmt.Sprintf("%s-report.xml", prefix), nil
+}
+
+func compileMatch(t *testing.T) *regexp.Regexp {
+	rx, err := regexp.Compile(*matchTest)
+	if err != nil {
+		t.Fatalf("Error compiling -match flag %q: %v", *matchTest, err)
+	}
+	return rx
+}