Review query escaping and add test coverage

[JanJakes]

Review strings escaping in the MySQL lexer and SQLite driver, and add a comprehensive test suite.

1. Verify string literal escaping (AST-based SQLite driver #1 (comment)).
2. Add a comprehensive test suite (AST-based SQLite driver #1 (comment)).
3. Consider unescaping MySQL queries in lexer (AST-based SQLite driver #1 (comment)).

[JanJakes]

See: WordPress/sqlite-database-integration#187