database/sql: add Drivers, returning list of registered drivers

Fixes #7969.

LGTM=bradfitz
R=bradfitz
CC=golang-codereviews
https://golang.org/cl/158950043

src/database/sql/fakedb_test.go

@@ -10,6 +10,7 @@ import (
 	"fmt"
 	"io"
 	"log"
+	"sort"
 	"strconv"
 	"strings"
 	"sync"
@@ -126,6 +127,27 @@ func init() {
 	Register("test", fdriver)
 }
 
+func contains(list []string, y string) bool {
+	for _, x := range list {
+		if x == y {
+			return true
+		}
+	}
+	return false
+}
+
+type Dummy struct {
+	driver.Driver
+}
+
+func TestDrivers(t *testing.T) {
+	Register("invalid", Dummy{})
+	all := Drivers()
+	if len(all) < 2 || !sort.StringsAreSorted(all) || !contains(all, "test") || !contains(all, "invalid") {
+		t.Fatalf("Drivers = %v, want sorted list with at least [invalid, test]", all)
+	}
+}
+
 // Supports dsn forms:
 //    <dbname>
 //    <dbname>;<opts>  (only currently supported option is `badConn`,