all: fix plugin import ordering (#1717)

Got a bit messed up with stb lib "context" usage.

plugin/erratic/erratic.go

@@ -2,13 +2,12 @@
 package erratic
 
 import (
+	"context"
 	"sync/atomic"
 	"time"
 
 	"github.com/coredns/coredns/request"
 
-	"context"
-
 	"github.com/miekg/dns"
 )
 

plugin/erratic/erratic_test.go

@@ -1,13 +1,12 @@
 package erratic
 
 import (
+	"context"
 	"testing"
 
 	"github.com/coredns/coredns/plugin/pkg/dnstest"
 	"github.com/coredns/coredns/plugin/test"
 
-	"context"
-
 	"github.com/miekg/dns"
 )