Check for content on STDIN is broken in Behat

[schlessera]

The check in WP_CLI\Entity\Utils\has_stdin() to see whether content is available from STDIN is currently breaking within the Behat environment.

Strangely enough, it seems to work outside of Behat.

This needs further examination.

Behat output (https://travis-ci.org/wp-cli/entity-command/jobs/412368071#L699-L706):

01. $ wp user import-csv -
    
    cwd: /tmp/wp-cli-test-run-user-import-csv.feature.142-5b673717041d62.79844176/
    run time: 0.28037309646606
    exit status: 0
    In step `Then STDERR should be:'.           # vendor/wp-cli/wp-cli-tests/features/steps/then.php:17
    From scenario `Importing users from STDIN'. # features/user-import-csv.feature:142
    Of feature `Import users from CSV'.         # features/user-import-csv.feature