database/sql: make TestDrivers not crash on second run

Using -test.cpu=1,1 made it crash before.

Fixes #9024

LGTM=iant
R=adg, iant
CC=golang-codereviews
https://golang.org/cl/169860043

src/database/sql/sql.go

@@ -37,6 +37,11 @@ func Register(name string, driver driver.Driver) {
 	drivers[name] = driver
 }
 
+func unregisterAllDrivers() {
+	// For tests.
+	drivers = make(map[string]driver.Driver)
+}
+
 // Drivers returns a sorted list of the names of the registered drivers.
 func Drivers() []string {
 	var list []string