storage/sql/postgres: expose stdlib tunables, set them for tests

- adapted TestUnmarshalConfig to ensure the fields are read in
- added a test to see that at least MaxOpenConns works:
  - this is only exposed through (*db).Stats() in go 1.11, so this test
    has a build tag
  - the other two configurables can't be read back, so we've got to
    trust that the mechanism works given the one instance that's tested..

Signed-off-by: Stephan Renatus <srenatus@chef.io>

cmd/dex/config_test.go

@@ -18,10 +18,14 @@ func TestUnmarshalConfig(t *testing.T) {
 	rawConfig := []byte(`
 issuer: http://127.0.0.1:5556/dex
 storage:
-  type: sqlite3
+  type: postgres
   config:
-    file: examples/dex.db
-
+    host: 10.0.0.1
+    port: 65432
+    maxOpenConns: 5
+    maxIdleConns: 3
+    connMaxLifetime: 30
+    connectionTimeout: 3
 web:
   http: 127.0.0.1:5556
 staticClients:
@@ -69,9 +73,14 @@ logger:
 	want := Config{
 		Issuer: "http://127.0.0.1:5556/dex",
 		Storage: Storage{
-			Type: "sqlite3",
-			Config: &sql.SQLite3{
-				File: "examples/dex.db",
+			Type: "postgres",
+			Config: &sql.Postgres{
+				Host:              "10.0.0.1",
+				Port:              65432,
+				MaxOpenConns:      5,
+				MaxIdleConns:      3,
+				ConnMaxLifetime:   30,
+				ConnectionTimeout: 3,
 			},
 		},
 		Web: Web{