dashboard: add some more mobile tests

They were already passing it turns out, but let's add these lines
specifically anyway.

Updates golang/go#30779

Change-Id: I00e0d3555f06b00098f63fc31bee58fd516c8a66
Reviewed-on: https://go-review.googlesource.com/c/build/+/167197
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Elias Naur <mail@eliasnaur.com>

dashboard/builders_test.go

@@ -333,6 +333,11 @@ func TestBuilderConfig(t *testing.T) {
 		{b("darwin-amd64-wikofever", "go"), notBuilder},
 		{b("darwin-amd64-wikofever", "net"), notBuilder},
 
+		// Mobile builders that run with GOOS=linux/darwin and have
+		// a device attached.
+		{b("linux-amd64-androidemu", "mobile"), both},
+		{b("darwin-amd64-wikofever", "mobile"), onlyPost},
+
 		// But the emulators run all:
 		{b("android-amd64-emu", "mobile"), isBuilder},
 		{b("android-386-emu", "mobile"), isBuilder},