add more tests

core/stat/internal/cpu_linux_test.go

@@ -1,6 +1,14 @@
 package internal
 
-import "testing"
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestRefreshCpu(t *testing.T) {
+	assert.True(t, RefreshCpu() >= 0)
+}
 
 func BenchmarkRefreshCpu(b *testing.B) {
 	for i := 0; i < b.N; i++ {