Partial test suite run builds corrupt test-in-container testroot

[DJ Delorie]

I did a "make" followed by "make libio/tests" and compared with a "make;
make check" and they produced identical testroots (including iconvconfig).

I tried a "./configure; make libio/tests" and it failed pretty quickly
(shell-container not found) since support/ hadn't been built.

Please provide more detailed reproduction instructions, as I don't see a
"corrupt testroot" yet.