Add a way to select the stream under test in system test runner

This provides a means to select a stream to test by input type.

Closes elastic#186

internal/testrunner/runners/system/runner.go

@@ -340,7 +340,8 @@ func createPackageDatastream(
 	ds packages.DataStreamManifest,
 	c testConfig,
 ) ingestmanager.PackageDataStream {
-	streamInput := ds.Streams[0].Input
+	stream := ds.Streams[getDataStreamIndex(c.Input, ds)]
+	streamInput := stream.Input
 	r := ingestmanager.PackageDataStream{
 		Name:      fmt.Sprintf("%s-%s", pkg.Name, ds.Name),
 		Namespace: "ep",
@@ -372,7 +373,7 @@ func createPackageDatastream(
 
 	// Add dataStream-level vars
 	dsVars := ingestmanager.Vars{}
-	for _, dsVar := range ds.Streams[0].Vars {
+	for _, dsVar := range stream.Vars {
 		val := dsVar.Default
 
 		cfgVar, exists := c.DataStream.Vars[dsVar.Name]
@@ -413,6 +414,17 @@ func createPackageDatastream(
 	return r
 }
 
+// getDataStreamIndex returns the index of the data stream whose input name
+// matches. Otherwise it returns the 0.
+func getDataStreamIndex(inputName string, ds packages.DataStreamManifest) int {
+	for i, s := range ds.Streams {
+		if s.Input == inputName {
+			return i
+		}
+	}
+	return 0
+}
+
 func deleteDataStreamDocs(esClient *es.Client, dataStream string) error {
 	body := strings.NewReader(`{ "query": { "match_all": {} } }`)
 	_, err := esClient.DeleteByQuery([]string{dataStream}, body)

internal/testrunner/runners/system/test_config.go

@@ -22,6 +22,7 @@ import (
 const configFileName = "config.yml"
 
 type testConfig struct {
+	Input      string                       `config:"input"`
 	Vars       map[string]packages.VarValue `config:"vars"`
 	DataStream struct {
 		Vars map[string]packages.VarValue `config:"vars"`