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/fakedb_test.go

@@ -141,6 +141,8 @@ type Dummy struct {
 }
 
 func TestDrivers(t *testing.T) {
+	unregisterAllDrivers()
+	Register("test", fdriver)
 	Register("invalid", Dummy{})
 	all := Drivers()
 	if len(all) < 2 || !sort.StringsAreSorted(all) || !contains(all, "test") || !contains(all, "invalid") {