provider/aws: Add an acceptance test that covers the new behaviour in

the `aws_iam_group_membership` resource

```
% make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSGroupMembership_'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
TF_ACC=1 go test ./builtin/providers/aws -v
-run=TestAccAWSGroupMembership_ -timeout 120m
=== RUN   TestAccAWSGroupMembership_basic
--- PASS: TestAccAWSGroupMembership_basic (74.14s)
=== RUN   TestAccAWSGroupMembership_paginatedUserList
--- PASS: TestAccAWSGroupMembership_paginatedUserList (273.29s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/aws    347.447s
```

builtin/providers/aws/resource_aws_iam_group_membership_test.go

@@ -57,6 +57,26 @@ func TestAccAWSGroupMembership_basic(t *testing.T) {
 	})
 }
 
+func TestAccAWSGroupMembership_paginatedUserList(t *testing.T) {
+	var group iam.GetGroupOutput
+
+	resource.Test(t, resource.TestCase{
+		PreCheck:     func() { testAccPreCheck(t) },
+		Providers:    testAccProviders,
+		CheckDestroy: testAccCheckAWSGroupMembershipDestroy,
+		Steps: []resource.TestStep{
+			resource.TestStep{
+				Config: testAccAWSGroupMemberConfigPaginatedUserList,
+				Check: resource.ComposeTestCheckFunc(
+					testAccCheckAWSGroupMembershipExists("aws_iam_group_membership.team", &group),
+					resource.TestCheckResourceAttr(
+						"aws_iam_group_membership.team", "users.#", "101"),
+				),
+			},
+		},
+	})
+}
+
 func testAccCheckAWSGroupMembershipDestroy(s *terraform.State) error {
 	conn := testAccProvider.Meta().(*AWSClient).iamconn
 
@@ -202,3 +222,22 @@ resource "aws_iam_group_membership" "team" {
 	group = "${aws_iam_group.group.name}"
 }
 `
+
+const testAccAWSGroupMemberConfigPaginatedUserList = `
+resource "aws_iam_group" "group" {
+	name = "test-paginated-group"
+	path = "/"
+}
+
+resource "aws_iam_group_membership" "team" {
+	name = "tf-testing-paginated-group-membership"
+	users = ["${aws_iam_user.user.*.name}"]
+	group = "${aws_iam_group.group.name}"
+}
+
+resource "aws_iam_user" "user" {
+        count = 101
+	name = "${format("paged-test-user-%d", count.index + 1)}"
+	path = "/"
+}
+`