geosolutions-it#10127: update tests to enable react-redux 7.x upwards

[fkellner]

Description

As described in https://github.com/geosolutions-it/MapStore2/issues/10127, this PR updates tests relying on 'render' returning a reference to enable react-redux 7.x upwards (where some components become stateless and render no longer returns a reference, even though it was successful).

Basically, for those tests, the indicator if the render function was successful is no longer that it returns a reference to a class instance, but that the DOM was modified.

For most tests, this basic check is complemented by more in-depth checks, which were left unchanged.

Rendering the MapViewerCmp did not result in measurable changes to the DOM (the content of the container div stayed an empty string), so instead, the new test checks that the onLoad callback function is being called.

What kind of change does this PR introduce? (check one with "x", remove the others)

• [x] Refactoring (no functional changes, no api changes)

Issue

What is the current behavior?

#10127

What is the new behavior?

Tests still run through regardless of whether react-redux 6.x or 7.x is being used.

Breaking change

Does this PR introduce a breaking change? (check one with "x", remove the other)

• [x] No

Other useful information