[Synthetics] Add data-test-subj ids (#188635)

## Summary

Add `data-test-subj` html properties for:
- mock IDP login button
- in 2 places for connectors

Required for defining synthetic tests for Serverless

packages/kbn-mock-idp-plugin/public/login_page.tsx

@@ -139,6 +139,7 @@ export const LoginPage = () => {
               actions={[
                 <EuiButton
                   type="submit"
+                  data-test-subj="loginButton"
                   disabled={!formik.isValid || !isRolesDefined()}
                   isLoading={formik.isSubmitting}
                   fill

x-pack/plugins/serverless_search/public/application/components/connectors/connector_config/connection_details_panel.tsx

@@ -60,7 +60,7 @@ export const ConnectionDetails: React.FC<ConnectionDetailsProps> = ({
             <strong>service_type</strong>
           </EuiText>
         </EuiFlexItem>
-        <EuiFlexItem grow={false}>
+        <EuiFlexItem grow={false} data-test-subj="serverlessSearchConnectorServiceType">
           {Boolean(serviceType) && <EuiCode>{serviceType}</EuiCode>}
         </EuiFlexItem>
       </EuiFlexGroup>

x-pack/plugins/serverless_search/public/application/components/connectors/edit_connector.tsx

@@ -126,6 +126,7 @@ export const EditConnector: React.FC = () => {
               >
                 <EuiContextMenu
                   initialPanelId={0}
+                  data-test-subj="serverlessSearchEditConnectorContextMenu"
                   panels={[
                     {
                       id: 0,