This is a followup from bug#60464 and friends. Over there, we discussed 
problems with Pcomplete when used from Eshell, namely that Eshell 
sometimes gives Pcomplete non-string arguments. I mentioned that we 
should probably have automated tests for the Eshell side so that we can 
make further improvements to Pcomplete without causing regressions, so 
here are some tests.

I also fixed an edge case in 'eshell-complete-parse-arguments' where it 
wasn't correctly handling the new variable-splicing syntax in Eshell. 
That's patch 0002.

Of course, these tests are just a start, and there are probably lots of 
others that we could add.