filebeat/input/filestream: Remove x-pack dependency (#40483)

To fix the failing linter rule that requires OSS code to not depend on x-pack
code, this change removes the dependency on x-pack/dockerlogbeat/pipelinemock
from the Filebeat filestream input. It does this by creating a mock client
that is local to filestream test code ("A little copying is better than a
little dependency").

Fixes #40293

filebeat/input/filestream/internal/input-logfile/harvester_test.go

@@ -31,9 +31,9 @@ import (
 
 	"github.com/elastic/beats/v7/filebeat/input/filestream/internal/task"
 	input "github.com/elastic/beats/v7/filebeat/input/v2"
+	"github.com/elastic/beats/v7/libbeat/beat"
 	"github.com/elastic/beats/v7/libbeat/common/atomic"
 	"github.com/elastic/beats/v7/libbeat/tests/resources"
-	"github.com/elastic/beats/v7/x-pack/dockerlogbeat/pipelinemock"
 	"github.com/elastic/elastic-agent-libs/logp"
 )
 
@@ -393,7 +393,7 @@ func TestDefaultHarvesterGroup(t *testing.T) {
 func testDefaultHarvesterGroup(t *testing.T, mockHarvester Harvester) *defaultHarvesterGroup {
 	return &defaultHarvesterGroup{
 		readers:    newReaderGroup(),
-		pipeline:   &pipelinemock.MockPipelineConnector{},
+		pipeline:   &MockPipeline{},
 		harvester:  mockHarvester,
 		store:      testOpenStore(t, "test", nil),
 		identifier: &sourceIdentifier{"filestream::.global::"},
@@ -465,3 +465,71 @@ func (tl *testLogger) Errorf(format string, args ...interface{}) {
 func (tl *testLogger) String() string {
 	return (*strings.Builder)(tl).String()
 }
+
+// MockClient is a mock implementation of the beat.Client interface.
+type MockClient struct {
+	published []beat.Event // Slice to store published events
+
+	closed bool       // Flag to indicate if the client is closed
+	mu     sync.Mutex // Mutex to synchronize access to the published events slice
+}
+
+// GetEvents returns all the events published by the mock client.
+func (m *MockClient) GetEvents() []beat.Event {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	return m.published
+}
+
+// Publish publishes a single event.
+func (m *MockClient) Publish(e beat.Event) {
+	es := make([]beat.Event, 1)
+	es = append(es, e)
+
+	m.PublishAll(es)
+}
+
+// PublishAll publishes multiple events.
+func (m *MockClient) PublishAll(es []beat.Event) {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	m.published = append(m.published, es...)
+}
+
+// Close closes the mock client.
+func (m *MockClient) Close() error {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	if m.closed {
+		return fmt.Errorf("mock already closed")
+	}
+
+	m.closed = true
+	return nil
+}
+
+// MockPipeline is a mock implementation of the beat.Pipeline interface.
+type MockPipeline struct {
+	c  beat.Client // Client used by the pipeline
+	mu sync.Mutex  // Mutex to synchronize access to the client
+}
+
+// ConnectWith connects the mock pipeline with a client using the provided configuration.
+func (mp *MockPipeline) ConnectWith(config beat.ClientConfig) (beat.Client, error) {
+	mp.mu.Lock()
+	defer mp.mu.Unlock()
+
+	c := &MockClient{}
+
+	mp.c = c
+
+	return c, nil
+}
+
+// Connect connects the mock pipeline with a client using the default configuration.
+func (mp *MockPipeline) Connect() (beat.Client, error) {
+	return mp.ConnectWith(beat.ClientConfig{})
+}