golinter fix (#1807)

This fix fixes golinter warning:
```
plugin/tls/tls_test.go:1::warning: file is not goimported (goimports)
```
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

plugin/tls/tls_test.go

@@ -14,8 +14,8 @@ func TestTLS(t *testing.T) {
 		expectedRoot       string // expected root, set to the controller. Empty for negative cases.
 		expectedErrContent string // substring from the expected error. Empty for positive cases.
 	}{
-	// positive
-	// negative
+		// positive
+		// negative
 	}
 
 	for i, test := range tests {