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.go

@@ -0,0 +1,98 @@
+package gojunitreport
+
+import (
+	"encoding/json"
+	"encoding/xml"
+	"fmt"
+	"io"
+	"os"
+	"time"
+
+	"github.com/jstemmer/go-junit-report/v2/gtr"
+	"github.com/jstemmer/go-junit-report/v2/junit"
+	"github.com/jstemmer/go-junit-report/v2/parser/gotest"
+)
+
+type parser interface {
+	Parse(r io.Reader) (gtr.Report, error)
+	Events() []gotest.Event
+}
+
+// Config contains the go-junit-report command configuration.
+type Config struct {
+	Parser        string
+	Hostname      string
+	PackageName   string
+	SkipXMLHeader bool
+	Properties    map[string]string
+	TimestampFunc func() time.Time
+
+	// For debugging
+	PrintEvents bool
+}
+
+// Run runs the go-junit-report command and returns the generated report.
+func (c Config) Run(input io.Reader, output io.Writer) (*gtr.Report, error) {
+	var p parser
+	switch c.Parser {
+	case "gotest":
+		p = gotest.NewParser(
+			gotest.PackageName(c.PackageName),
+			gotest.TimestampFunc(c.TimestampFunc),
+		)
+	case "gojson":
+		p = gotest.NewJSONParser(
+			gotest.PackageName(c.PackageName),
+			gotest.TimestampFunc(c.TimestampFunc),
+		)
+	default:
+		return nil, fmt.Errorf("invalid parser: %s", c.Parser)
+	}
+
+	report, err := p.Parse(input)
+	if err != nil {
+		return nil, fmt.Errorf("error parsing input: %w", err)
+	}
+
+	if c.PrintEvents {
+		enc := json.NewEncoder(os.Stderr)
+		for _, event := range p.Events() {
+			if err := enc.Encode(event); err != nil {
+				return nil, err
+			}
+		}
+	}
+
+	for i := range report.Packages {
+		for k, v := range c.Properties {
+			report.Packages[i].SetProperty(k, v)
+		}
+	}
+
+	if err = c.writeXML(output, report); err != nil {
+		return nil, err
+	}
+	return &report, nil
+}
+
+func (c Config) writeXML(w io.Writer, report gtr.Report) error {
+	testsuites := junit.CreateFromReport(report, c.Hostname)
+
+	if !c.SkipXMLHeader {
+		_, err := fmt.Fprintf(w, xml.Header)
+		if err != nil {
+			return err
+		}
+	}
+
+	enc := xml.NewEncoder(w)
+	enc.Indent("", "\t")
+	if err := enc.Encode(testsuites); err != nil {
+		return err
+	}
+	if err := enc.Flush(); err != nil {
+		return err
+	}
+	_, err := fmt.Fprintf(w, "\n")
+	return err
+}