testing: reduce verbosity of output log

Previously the log output included the test name twice and a long date
format. The test output is already grouped by test, so adding the test
name did not add any new information. The date and time are only useful
to understand elapsed time, so using a short format should provide
succident detail.

Also fixed a bug in NewTestAgentWithFields where nil was returned
instead of the test agent.

agent/agent_test.go

@@ -138,11 +138,7 @@ func TestAgent_ConnectClusterIDConfig(t *testing.T) {
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			// This is a rare case where using a constructor for TestAgent
-			// (NewTestAgent and the likes) won't work, since we expect an error
-			// in one test case, and the constructors have built-in retry logic
-			// that runs automatically upon error.
-			a := &TestAgent{Name: tt.name, HCL: tt.hcl, LogOutput: testutil.TestWriter(t)}
+			a := NewTestAgentWithFields(t, false, TestAgent{HCL: tt.hcl})
 			err := a.Start()
 			if tt.wantErr {
 				if err == nil {

agent/testagent.go

@@ -25,7 +25,6 @@ import (
 	"github.com/hashicorp/consul/agent/structs"
 	"github.com/hashicorp/consul/api"
 	"github.com/hashicorp/consul/sdk/freeport"
-	"github.com/hashicorp/consul/sdk/testutil"
 	"github.com/hashicorp/consul/sdk/testutil/retry"
 )
 
@@ -104,11 +103,8 @@ func NewTestAgentWithFields(t *testing.T, start bool, ta TestAgent) *TestAgent {
 	if a.Name == "" {
 		a.Name = t.Name()
 	}
-	if a.LogOutput == nil {
-		a.LogOutput = testutil.TestWriter(t)
-	}
 	if !start {
-		return nil
+		return &a
 	}
 
 	retry.RunWith(retry.ThreeTimes(), t, func(r *retry.R) {
@@ -162,9 +158,9 @@ func (a *TestAgent) Start() (err error) {
 	}
 
 	logger := hclog.NewInterceptLogger(&hclog.LoggerOptions{
-		Name:   a.Name,
-		Level:  hclog.Debug,
-		Output: logOutput,
+		Level:      hclog.Debug,
+		Output:     logOutput,
+		TimeFormat: "04:05.000",
 	})
 
 	portsConfig, returnPortsFn := randomPortsSource(a.UseTLS)