[REF] extract buildClause from CRM_Report_Form_Event_Income

[REF] extract buildClause from CRM_Report_Form_Event_Income

This report is currently excluded from unit tests by virtue of it being declared as skipped in
api_v3_ReportTemplateTest

We have a PR https://github.com/civicrm/civicrm-core/pull/13928 to alter test logic.

I'm really not comfortable making any logic changes to the report until
it is under testing. Extracting buildQuery removes one of the blockers to
bringing it under testing (remaining are moving most / all logic from
postProcess to beginPostProcessCommon and fixing up the setPager function -
which probably just means removing it