fix: Notice: A non well formed numeric value

[domi41]

… encountered in features/bootstrap/FeatureContext.php line 66

On PHP 7.4

microtime()'s output was a string, and we multipied it, hence the notice.

Not 100% sure this fix works on earlier versions of PHP. Online doc https://www.php.net/manual/en/function.microtime.php suggests that it should work.

See #12