dashboard: add "longtest" builder to run all tests, without -short mode

Fixes golang/go#12508

Change-Id: I62e1e296f10c7622c1967a5530c4b0e07c136dfd
Reviewed-on: https://go-review.googlesource.com/113436
Reviewed-by: Bryan C. Mills <bcmills@google.com>

dashboard/builders.go

@@ -1083,6 +1083,13 @@ func init() {
 		MaxAtOnce: 1,
 		Notes:     "Debian Stretch",
 	})
+	addBuilder(BuildConfig{
+		Name:      "linux-amd64-longtest",
+		HostType:  "host-linux-stretch",
+		MaxAtOnce: 1,
+		Notes:     "Debian Stretch with go test -short=false",
+		env:       []string{"GO_TEST_SHORT=0"},
+	})
 	addBuilder(BuildConfig{
 		Name:              "linux-arm",
 		HostType:          "host-linux-arm-scaleway",