Fix SQLite skipped tests

[jasonvarga]

Testbench 7.12.1 introduces a change where if the sqlite file doesn't exist, it'll change the database.default config to their custom testing connection.

In our tests, we were checking for whether database.default was sqlite, which now be false, so the skipped tests would run, and cause an error.

This PR changes the condition to look for the driver of the appropriate database connection instead.

[what-the-diff]

• Replaced config('database.default') === 'sqlite' with $this->isUsingSqlite()
• Added isUsingSqlite method to TestCase class