connectors: refactor filter code into a helper package

I hope I didn't miss any :D

Signed-off-by: Stephan Renatus <srenatus@chef.io>

pkg/groups/groups_test.go

@@ -0,0 +1,26 @@
+package groups_test
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+
+	"github.com/dexidp/dex/pkg/groups"
+)
+
+func TestFilter(t *testing.T) {
+	cases := map[string]struct {
+		given, required, expected []string
+	}{
+		"nothing given":                 {given: []string{}, required: []string{"ops"}, expected: []string{}},
+		"exactly one match":             {given: []string{"foo"}, required: []string{"foo"}, expected: []string{"foo"}},
+		"no group of the required ones": {given: []string{"foo", "bar"}, required: []string{"baz"}, expected: []string{}},
+		"subset matching":               {given: []string{"foo", "bar", "baz"}, required: []string{"bar", "baz"}, expected: []string{"bar", "baz"}},
+	}
+	for name, tc := range cases {
+		t.Run(name, func(t *testing.T) {
+			actual := groups.Filter(tc.given, tc.required)
+			assert.ElementsMatch(t, tc.expected, actual)
+		})
+	}
+}