testing: include prefix in TestCmdline tests

Will become relevant with the next commit, which moves the
source name from being part of the text into the prefix. So far,
the prefix is unused during Cmdline tests.

src/syncevo/Cmdline.cpp

@@ -4535,6 +4535,9 @@ private:
                     out << "[" << levelToStr(level) << "] ";
                     m_all << "[" << levelToStr(level) << "] ";
                 }
+                if (options.m_prefix) {
+                    out << options.m_prefix << " :";
+                }
                 out << str;
                 m_all << str;
                 if (!boost::ends_with(str, "\n")) {