fix(sqlite3): resolve relative test database paths

[fzlzjerry]

Summary

• resolve relative SQLite dbname values against the directory containing the loaded SQLBoiler config
• preserve the existing behavior for absolute paths and configurations without a config file
• add an end-to-end regression that generates models from a relative SQLite database and runs the generated tests from the output directory

Testing

• go test -count=1 ./drivers/sqlboiler-sqlite3/driver
• go build ./...
• go vet ./...

Closes #1409

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