Add Pacts tests for Support API

We expect a 201 success if the parameters provided for the
`raise_support_ticket` are valid, and a 422 error if not.

When these tests are run they output a JSON pactfile, which is
published to our [pact broker](https://github.com/alphagov/govuk-pact-broker).

test/pacts/support_api_pact_test.rb

@@ -0,0 +1,69 @@
+require "test_helper"
+require "gds_api/support_api"
+
+describe "GdsApi::SupportApi pact tests" do
+  include PactTest
+
+  describe "#raise_support_ticket" do
+    let(:api_client) { GdsApi::SupportApi.new(support_api_host) }
+
+    it "responds with a 201 Success if the parameters provided are valid" do
+      support_api
+        .given("the parameters are valid")
+        .upon_receiving("a raise ticket request")
+        .with(
+          method: :post,
+          path: "/support-tickets",
+          headers: GdsApi::JsonClient.default_request_with_json_body_headers,
+          body: {
+            subject: "Feedback for app",
+            tags: %w[app_name],
+            user_agent: "Safari",
+            description: "There is something wrong with this page.",
+          },
+        )
+        .will_respond_with(
+          status: 201,
+          body: {
+            status: "success",
+          },
+          headers: {
+            "Content-Type" => "application/json; charset=utf-8",
+          },
+        )
+
+      api_client.raise_support_ticket(
+        subject: "Feedback for app",
+        tags: %w[app_name],
+        user_agent: "Safari",
+        description: "There is something wrong with this page.",
+      )
+    end
+
+    it "responds with 422 Error when required parameters are not provided" do
+      support_api
+      .given("the required parameters are not provided")
+      .upon_receiving("a raise ticket request")
+      .with(
+        method: :post,
+        path: "/support-tickets",
+        headers: GdsApi::JsonClient.default_request_with_json_body_headers,
+        body: {
+          subject: "Ticket without body",
+        },
+      )
+      .will_respond_with(
+        status: 422,
+        body: {
+          status: "error",
+        },
+      )
+
+      begin
+        api_client.raise_support_ticket(subject: "Ticket without body")
+      rescue GdsApi::HTTPUnprocessableEntity
+        # expected outcome
+      end
+    end
+  end
+end