Prevent regression of closed issues

Problem/Motivation

Field creation and entity test are covered.
Closed issues should also be covered (at least bug reports) to prevent regressions.
New issues should contain tests.

Proposed resolution

Use PHPUnit and add a base class for the setup / field creation.

Remaining tasks

Issues that should be covered:

• #2921349: Serial field value not in HTTP API responses (not in JSON:API, not in REST)
• #3058900: Initialize values on existing entities for other entity types than Node
• #2847954: Allow having starting value for the serial field and set existing entities initialization as optional
• #2849371: Not able to get calculated value in hook_node_presave() or hook_node_insert()
• #2868303: Type not found for custom Entity
• #2834714: Serial module field tables not created during config synchronization