ftrace: fix error with only one actor and add corresponding test case

When system has only one actor and call method trappy.summary_plots, it will report failure "zip argument #1 must support iteration".

This failure is cause by parsing allfreqs plots. Python always pass by value, so when there have only one actor the axis will pass by value and it's not a array type anymore. So finally zip will report it cannot support iteration. So in this case convert axis to array type.

Also per Javi's request, Javi shared one test case for it, so also commit test case for it.