CRM-15104 - Rewrote the unit test.

I think Jenkins didn't like my unit test. It showed this
message:

PHP Parse error:  syntax error, unexpected '[' in
/srv/buildkit/build/build-3/sites/all/modules/civicrm/tests/phpunit/api/v3/ParticipantTest.php
on line 492

I rewrote the unit test with less '[' on that line. Hoping that Jenkins
will be happy now :-)

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* CRM-15104: Line item issues when creating a participant using the API
  https://issues.civicrm.org/jira/browse/CRM-15104