[FEATURE] Migrate plugin away from using enzymejs for its unit tests.

[AWSHurneyt]

Is your feature request related to a problem?
Bumping the version of OpenSearch-Dashboards used by the alerting-dashboards-plugin to run unit tests from 1.3 to 2.0 caused the following 4 unit tests to start failing:

1. DelayedLoader.test.js should set Timer for 1 seconds if initial loading is true
2. DelayedLoader.test.js should clear Timer on componentWillUnmount if exists
3. DelayedLoader.test.js should not show loader if data fetching is finished before threshold
4. WhereExpression.test.js calls closeExpression when closing popover: This test examines the behavior of closing an EuiPopover component. OSD 2.0 bumped the version of EUI used by the plugins from 29.3.2 to 34.6.0. The changelog mentions that changes were made to the functionality of EuiPopover, and this PR describes changes that relate to handling escaping from a popover which impacted this test. This change seems to impact the way this component behaves when testing using enzymejs.

What solution would you like?
The alerting-dashboards-plugin should likely migrate away from using enzymejs to align with the other frontend plugins.

[AWSHurneyt]

Test 4 mentioned above has been set to skip for now.

[AWSHurneyt]

Refactoring the testing architecture in this way will be a backlog enhancement.