[Multiple Datasource Test] Add UT for custom_database_icon

[yujin-emma]

Is your feature request related to a problem? Please describe.

Add UT for custom_database_icon: https://app.codecov.io/gh/opensearch-project/OpenSearch-Dashboards/tree/main/src%2Fplugins%2Fdata_source_management%2Fpublic%2Fcomponents%2Fcustom_database_icon

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Add UT for custom_database_icon Describe alternatives you've considered

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