Register postgre driver in pkg/event/target (#6689)

Commit 5c13765168 removed postgre registration triggerd
by the automatic gofmt command but it was the only where pg is registered. This commit
fixes behavior and adds unit tests to check whether postgre & sql are registered or not.

pkg/event/target/postgresql.go

@@ -62,6 +62,8 @@ import (
 	"strings"
 	"time"
 
+	_ "github.com/lib/pq" // Register postgres driver
+
 	"github.com/minio/minio/pkg/event"
 	xnet "github.com/minio/minio/pkg/net"
 )