[FEATURE] Add set default data source test in data source management

[BionIT]

Is your feature request related to a problem?

In Opensearch dashboards, we introduced default data source concept, which should be the default data source when no action option is passed in. We need to add tests to allow setting the default data source, and test if devtools and/or add sample data page has it set by default

What solution would you like?

Add set default data source test in data source management

What alternatives have you considered?

Rely on manual tests

Do you have any additional context?

Add any other context or screenshots about the feature request here.

[kavilla]

@BionIT should we move this to 2.15?

[kavilla]

Checked with @BionIT, it is for 2.15.