Local Environment: Add tests for first and second run step behaviors for each mode

[danielbachhuber]

Some modes configure a SQLite database, and others do not. We should have some tests covering should happen for the first and second run of each mode. The tests should also verify the server can start as expected.

Related #302 (comment) WordPress/playground-tools#26 #310

Some PRs that should have tests:

• wp-now: Abstract downloading and loading mu-plugins #319
• wp-now: Mount db.php file instead of copying it #318
• wp-now: Mount SQLite database directory from wp-content stored in .wp-now #320
• wp-now: Only apply sqlite for 'wp-content' when missing wp-config.php #310

Done is:

• Each mode has explicit test coverage for the behaviors of the first and second modes.

[danielbachhuber]

🚢