ESQL: re-apply the parallel loading of test data

[astefan]

Description

Re-apply the changes that made parallel loading of data for the Spec tests, meaning reverting this #134738 and, possibly this one #134511

After an internal investigation from @craigtaverner, it is possible that a bug while creating the data is hidden. This can potentially aggravate a race condition in the setup() method in EsqlSpecTestCase where the dataLoaded boolean is not set to true even if the loading happens. If later parts of the loading process fail, the data will have been loaded, while the boolean remains false, so the next test will re-attempt the data load, leading to the error that shows that a resource already exists - resource_already_exists_exception.

Related to #131949

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)

[craigtaverner]

The prototype PR at #134995 has a refactored index setup() method that might protect against the issues seen here. I'll investigate using it to get this working.