* refactor: simply browserslists defaults overwriting
When `hasTargets` is `false`, `targets` is always an empty object by definition, `objectToBrowserslist({})` always return `[]` therefore we can remove `objectToBrowserslist`.
* test: apply external-helpers on preset-env test fixtures
* test: add ie 6 to force all transforms in corejs 3, debug, corejs 2 fixtures
* Add ie 6 as targets for test fixtures
* Add support for logs to transform-fixture-test-runner
* Test some warnings
* Use stderr/stdout, and add a validateLogs test option
* Normalize CWD in logs and rename vars
* Silence preset-env tests
* Restore old methods
