database/sql: run tests with different driver variants

Database drivers can support a variety of optional features.
For example, a driver.Conn can optionally implement the
Queryer and/or QueryerContext interfaces.

We currently run most database/sql tests with a general-purpose
test driver which implements most optional interfaces.

Add support for running tests with a variety of different drivers.
To start with, use two variations: A "basic" driver which only
implements the minimal mandatory interfaces, and a "default"
driver which implements most optional interfaces.

For #67546

Change-Id: I60bd02260810b6468e6b02a1ac7544406a6a6964
Reviewed-on: https://go-review.googlesource.com/c/go/+/766700
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/database/sql/fakedb_test.go

@@ -62,8 +62,21 @@ type fakeConnector struct {
 
 func (c *fakeConnector) Connect(context.Context) (driver.Conn, error) {
 	conn, err := fdriver.Open(c.name)
+	if err != nil {
+		return nil, err
+	}
 	conn.(*fakeConn).waiter = c.waiter
-	return conn, err
+	return conn, nil
+}
+
+func getFakeConn(c driver.Conn) *fakeConn {
+	return c.(interface {
+		getFakeConn() *fakeConn
+	}).getFakeConn()
+}
+
+func (c *fakeConn) getFakeConn() *fakeConn {
+	return c
 }
 
 func (c *fakeConnector) Driver() driver.Driver {