Make sure that oscap_init is run before each test case

Addressing:
[13/23] TestSessionDS#test_session_export_html_reportxmlXPathCompOpEval: function node-set not found
XPath error : Unregistered function
runtime error: file /usr/share/openscap/xsl/xccdf-report.xsl line 84 element variable
Failed to evaluate the expression of variable 'last_test_time'.