feat: automatically SetMode to TestMode when run go test. (#3139)

related issue: https://github.com/gin-gonic/gin/issues/3134

mode_test.go

@@ -5,6 +5,7 @@
 package gin
 
 import (
+	"flag"
 	"os"
 	"testing"
 
@@ -21,9 +22,16 @@ func TestSetMode(t *testing.T) {
 	assert.Equal(t, TestMode, Mode())
 	os.Unsetenv(EnvGinMode)
 
+	SetMode("")
+	assert.Equal(t, testCode, ginMode)
+	assert.Equal(t, TestMode, Mode())
+
+	tmp := flag.CommandLine
+	flag.CommandLine = flag.NewFlagSet("", flag.ContinueOnError)
 	SetMode("")
 	assert.Equal(t, debugCode, ginMode)
 	assert.Equal(t, DebugMode, Mode())
+	flag.CommandLine = tmp
 
 	SetMode(DebugMode)
 	assert.Equal(t, debugCode, ginMode)