[DOCS] Re-enable code snippet testing for machine learning

[lcawl]

Related to #30665, #31118, #33248, and #48663

The code snippets in the machine learning documentation was disabled because they ceased working after they were moved to the docs folder.

For example, the following error occurs:

1> [2018-09-07T20:27:20,562][INFO ][o.e.s.DocsClientYamlTestSuiteIT] [test]

Throwable #1: java.lang.RuntimeException: Failure at [reference/ml/apis/close-job:64]: rest api [xpack.ml.put_job] doesn't exist in the rest spec
at __randomizedtesting.SeedInfo.seed([D147CAFC346CBEEA:5913F5269A90D312]:0)

The following APIs still have testing disabled:

• Add events to calendar
• Add jobs to calendar
• Close jobs: [DOCS] Re-enable code snippet testing in close anomaly detection job API #48259
• Create anomaly detection job. Disabled due to See also [CI] Internal data frame indices can cause unrelated docs tests to fail #43271
• Create calendar
• Create data feeds. Before it can be re-enabled, the setup needs to be updated since it uses outdated types.
• Create filter
• Create data frame analytics job
• Delete calendar
• Delete datafeeds
• Delete events from calendar
• Delete filter
• Delete forecast
• Delete jobs
• Delete calendar jobs
• Delete model snapshots
• Delete data frame analytics [DOCS] Enables testing for data frame analytics APIs #44018 hits [ml.put_data_frame_analytics] doesn't exist in the rest spec error
• Evaluate data frame analytics
• Flush jobs
• Forecast Requires ability to wait for results
• Get calendars
• Get buckets
• Get overall buckets
• Get categories
• Get datafeeds
• Get datafeed stats
• Get influencers
• Get jobs
• Get job stats
• Get model snapshots
• Get calendar events
• Get filters
• Get records
• Get data frame analytics jobs
• Get data frame analytics job stats
• Open jobs
• Post data to job Intentionally omitted?
• Preview datafeeds
• Revert snapshots
• Start datafeed
• Start data frame analytics job
• Stop datafeed
• Stop data frame analytics job
• Update datafeed
• Update filter
• Update job
• Update model snapshot

Ideally at the same time as we get them working, we can update the examples to use one of the Kibana sample data sets too.

[elasticmachine]

Pinging @elastic/ml-core

[wchrisdean]

[doc issue triage]