Helpers: fixing test failures

[tombuildsstuff]

Fixing a test failure identified whilst running the tests for #1409 - fixes this:

------- Stdout: -------
=== RUN   TestAccAzureRMVirtualNetworkGateway_vpnClientConfig
--- FAIL: TestAccAzureRMVirtualNetworkGateway_vpnClientConfig (0.18s)
    testing.go:513: Step 0 error: config is invalid: azurerm_virtual_network_gateway.test: "vpn_client_configuration.0.radius_server_address" is not a valid IP4 address: '"1.2.3.4"`
FAIL

The validation functions also now pass:

$ go test -v ./azurerm/helpers/validate/
=== RUN   TestHelper_Validate_Ip4Address
--- PASS: TestHelper_Validate_Ip4Address (0.00s)
=== RUN   TestHelper_Validate_MacAddress
--- PASS: TestHelper_Validate_MacAddress (0.00s)
=== RUN   TestHelper_Validate_RFC3339Time
--- PASS: TestHelper_Validate_RFC3339Time (0.00s)
=== RUN   TestHelper_Validate_Rfc3339DateInFutureBy
--- PASS: TestHelper_Validate_Rfc3339DateInFutureBy (0.00s)
PASS
ok  	github.com/terraform-providers/terraform-provider-azurerm/azurerm/helpers/validate	0.027s

[tombuildsstuff]

The VPN Gateway year passes:

[TestAccAzureRMVirtualNetworkGateway_vpnClientConfig] [Test Output]
=== RUN   TestAccAzureRMVirtualNetworkGateway_vpnClientConfig
--- PASS: TestAccAzureRMVirtualNetworkGateway_vpnClientConfig (1908.03s)
PASS

[ghost]

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