ENH Let unit tests manipulate the DB before loading fixtures

[GuySartorelli]

Description

Fixtures in unit tests can be set up using an explicit table name - but there's no way (that I can see) to actually create the table in the time between the test DB being created and fixtures being loaded.

This PR adds a way for tests to do that.

See https://github.com/silverstripe/silverstripe-linkfield/pull/253 for an example of how this works.

Issues

• https://github.com/silverstripe/silverstripe-linkfield/issues/229

Pull request checklist

• [x] The target branch is correct
  • See picking the right version
• [x] All commits are relevant to the purpose of the PR (e.g. no debug statements, unrelated refactoring, or arbitrary linting)
  • Small amounts of additional linting are usually okay, but if it makes it hard to concentrate on the relevant changes, ask for the unrelated changes to be reverted, and submitted as a separate PR.
• [x] The commit messages follow our commit message guidelines
• [x] The PR follows our contribution guidelines
• [x] Code changes follow our coding conventions
• [x] This change is covered with tests (or tests aren't necessary for this change)
• [x] Any relevant User Help/Developer documentation is updated; for impactful changes, information is added to the changelog for the intended release
• [x] CI is green

[GuySartorelli]

Added a note that it is intended to be overridden by subclasses. There's also an associated docs PR now linked in the main issue.

[GuySartorelli]

rebased so the installer CI run was happy (see link in issue)

[michalkleiner]

Merge when needed.