Reenable serverless Discover CSV tests (elastic#180113)

## Summary

This PR reenables the serverless Discover CSV tests.

### Details

* The tests were failing for a new serverless ES image due to a change
in default shards
* The tests have been actually fixed in elastic#179901
* Due to the way how the ES pipeline picks up Kibana changes for their
verification, it took a while for this fix to take effect (actually
needed to wait for the next daily scheduled run)
* In the mean time, the test appeared to still be failing, so it was
skipped entirely, see elastic#180010
* With the ES serverless promotion now done, the skipped (and already
fixed) test suite can be reenabled

Closes elastic#180010

x-pack/test_serverless/api_integration/test_suites/common/reporting/generate_csv_discover.ts

@@ -160,8 +160,7 @@ export default ({ getService }: FtrProviderContext) => {
       });
     });
 
-    // FAILING ES PROMOTION SERVERLESS: https://github.com/elastic/kibana/issues/180010
-    describe.skip('with unmapped fields', () => {
+    describe('with unmapped fields', () => {
       before(async () => {
         await esArchiver.load(archives.unmappedFields.data);
         await kibanaServer.importExport.load(archives.unmappedFields.savedObjects);