daemon/logger/loggertest: expand log-follow tests

Following logs with a non-negative tail when the container log is empty
is broken on the journald driver when used with systemd 255. Add tests
which cover this edge case to our loggertest suite.

Signed-off-by: Cory Snider <csnider@mirantis.com>
(cherry picked from commit 931568032a)
Signed-off-by: Cory Snider <csnider@mirantis.com>

daemon/logger/loggertest/logreader.go

@@ -1,6 +1,7 @@
 package loggertest // import "github.com/docker/docker/daemon/logger/loggertest"
 
 import (
+	"fmt"
 	"runtime"
 	"strings"
 	"sync"
@@ -194,28 +195,31 @@ func (tr Reader) testTailEmptyLogs(t *testing.T, live bool) {
 func (tr Reader) TestFollow(t *testing.T) {
 	// Reader sends all logs and closes after logger is closed
 	// - Starting from empty log (like run)
-	t.Run("FromEmptyLog", func(t *testing.T) {
-		t.Parallel()
-		l := tr.Factory(t, logger.Info{
-			ContainerID:   "followstart0",
-			ContainerName: "logloglog",
-		})(t)
-		lw := l.(logger.LogReader).ReadLogs(logger.ReadConfig{Tail: -1, Follow: true})
-		defer lw.ConsumerGone()
+	for i, tail := range []int{-1, 0, 1, 42} {
+		i, tail := i, tail
+		t.Run(fmt.Sprintf("FromEmptyLog/Tail=%d", tail), func(t *testing.T) {
+			t.Parallel()
+			l := tr.Factory(t, logger.Info{
+				ContainerID:   fmt.Sprintf("followstart%d", i),
+				ContainerName: fmt.Sprintf("logloglog%d", i),
+			})(t)
+			lw := l.(logger.LogReader).ReadLogs(logger.ReadConfig{Tail: tail, Follow: true})
+			defer lw.ConsumerGone()
 
-		doneReading := make(chan struct{})
-		var logs []*logger.Message
-		go func() {
-			defer close(doneReading)
-			logs = readAll(t, lw)
-		}()
+			doneReading := make(chan struct{})
+			var logs []*logger.Message
+			go func() {
+				defer close(doneReading)
+				logs = readAll(t, lw)
+			}()
 
-		mm := makeTestMessages()
-		expected := logMessages(t, l, mm)
-		assert.NilError(t, l.Close())
-		<-doneReading
-		assert.DeepEqual(t, logs, expected, compareLog)
-	})
+			mm := makeTestMessages()
+			expected := logMessages(t, l, mm)
+			assert.NilError(t, l.Close())
+			<-doneReading
+			assert.DeepEqual(t, logs, expected, compareLog)
+		})
+	}
 
 	t.Run("AttachMidStream", func(t *testing.T) {
 		t.Parallel()