The thread plan list test is failing at least on Ubuntu Linux.

The thread plan list test is failing at least on Ubuntu Linux.
Mark it expected fail for now.

The test output shows that the "internal" thread listing isn't showing the
step out plan that we use to step back out of a function we're stepping into.
The internal plan listing code has nothing platform specific in it, so that
isn't the problem.

I am pretty sure the difference is that on MacOS we step into the function and then need to
step back out again so we push the internal plan the test is checking for.  But on Linux we
are able to step past the function without stepping into it.

So nothing is actually going wrong here, I just need to find a better test case where I
can ensure we are going to have to push a private plan.  It's probably better to test this
using a custom thread plan, then I can control the state of the plan stack better.

That's for Monday...