bamboo support option for nunit report format..

currently there seems to be no support for optionally setting a flag in unit test execution, to produce a nunit v2 report. this breaks bamboo use, due to this workaround documented https://jira.atlassian.com/browse/BAM-12433?focusedCommentId=855781&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-855781 on atlassian website.

if this is acceptable, would like to add a optional flag which will allow the parameter ";format=nunit2" to be added.
I hope i haven't missed anything obvious..

Start by tracing the unit test execution entry point in Cake.Recipe and how report-format options are passed through. Confirm the optional Bamboo-related flag produces the requested nunit2 parameter, then verify the existing test or execution checks cover both the default behavior and the new option.

Written by the indexing model from the issue text.