d/logger/loggertest: improve TestConcurrent

- Check the return value when logging messages
- Log the stream (stdout/stderr) and list of messages that were not read
- Wait until the logger is closed before returning early (panic/fatal)

Signed-off-by: Cory Snider <csnider@mirantis.com>

daemon/logger/loggertest/logreader.go

@@ -11,6 +11,7 @@ import (
 	"github.com/google/go-cmp/cmp"
 	"github.com/google/go-cmp/cmp/cmpopts"
 	"gotest.tools/v3/assert"
+	is "gotest.tools/v3/assert/cmp"
 	"gotest.tools/v3/assert/opt"
 
 	"github.com/docker/docker/api/types/backend"
@@ -437,7 +438,7 @@ func (tr Reader) TestConcurrent(t *testing.T) {
 	logAll := func(msgs []*logger.Message) {
 		defer wg.Done()
 		for _, m := range msgs {
-			l.Log(copyLogMessage(m))
+			assert.Check(t, l.Log(copyLogMessage(m)), "failed to log message %+v", m)
 		}
 	}
 
@@ -450,6 +451,15 @@ func (tr Reader) TestConcurrent(t *testing.T) {
 		defer l.Close()
 		wg.Wait()
 	}()
+	defer func() {
+		// Make sure log gets closed before we return
+		// so the temporary dir can be deleted
+		select {
+		case <-time.After(10 * time.Second):
+			t.Fatal("timed out waiting for logger to close")
+		case <-closed:
+		}
+	}()
 
 	// Check if the message count, order and content is equal to what was logged
 	for {
@@ -473,12 +483,8 @@ func (tr Reader) TestConcurrent(t *testing.T) {
 		*messages = (*messages)[1:]
 	}
 
-	assert.Equal(t, len(stdoutMessages), 0)
-	assert.Equal(t, len(stderrMessages), 0)
-
-	// Make sure log gets closed before we return
-	// so the temporary dir can be deleted
-	<-closed
+	assert.Check(t, is.Len(stdoutMessages, 0), "expected stdout messages were not read")
+	assert.Check(t, is.Len(stderrMessages, 0), "expected stderr messages were not read")
 }
 
 // logMessages logs messages to l and returns a slice of messages as would be