testutil: Set the environment variable NOLOGBUFFER=1 to have test agent logs go straight to stdout

This is a followup to the testing feature added in #5304.

While it is very nice to have test agent logs to through testing.T.Logf
so that only logs from failed tests are emitted, if a test is hanging or
taking a long time those logs are delayed. In that case setting
NOLOGBUFFER=1 will temporarily redirect them to stdout so you can see
immediate feedback about hangs when running the tests in verbose mode.

One current example of a test where this can be relevant is:

    $ go test ./agent/consul -run 'TestCatalog_ListServices_Stale' -v

testutil/testlog.go

@@ -1,12 +1,20 @@
 package testutil
 
 import (
+	"fmt"
 	"io"
 	"log"
+	"os"
 	"strings"
 	"testing"
 )
 
+var sendTestLogsToStdout bool
+
+func init() {
+	sendTestLogsToStdout = os.Getenv("NOLOGBUFFER") == "1"
+}
+
 func TestLogger(t testing.TB) *log.Logger {
 	return log.New(&testWriter{t}, "test: ", log.LstdFlags)
 }
@@ -25,6 +33,10 @@ type testWriter struct {
 
 func (tw *testWriter) Write(p []byte) (n int, err error) {
 	tw.t.Helper()
+	if sendTestLogsToStdout {
+		fmt.Fprint(os.Stdout, strings.TrimSpace(string(p))+"\n")
+	} else {
 		tw.t.Log(strings.TrimSpace(string(p)))
+	}
 	return len(p), nil
 }