SQLite: Support PHPUnit test suite

[adamziel]

Description

#547 added SQLite support through the aaemnnosttv/wp-sqlite-db drop-in database plugin which rewrites MySQL queries to SQLite queries. Unfortunately, many MySQL queries are unsupported at the moment and a lot of WordPress unit tests fail.

Let's find a way to support all the MySQL queries that can be translated to SQLite.

cc @aristath @felixarntz

[felixarntz]

The SQLite module will be removed from Performance Lab in the upcoming release, see #737 for additional context. As such, please report this issue in the standalone plugin repository instead.

[adamziel]

@felixarntz GitHub has a Transfer Issue feature which allows you to move issues to other repositories without losing the conversation or having to re-create them. It would be handy for keeping a continuity of all the SQLite-related issues. cc @aristath