TS-1365 Fix AIO unit test
TS-2467: traffic_shell doesn't work with tcl 8.6, works with 8.5

Segfault in libtcl8.6.so when calling Tcl_SetMainLoop(Tcl_ReadlineMain).
If setting Tcl MainLoop, we should call Tcl_FindExecutable(NULL),
which fixes TCL 8.6 and should also work with older versions.