test_fdio assumes the two lowest free descriptors are free #63

tests/test_fdio.c:125 checks !is_open(before) && !is_open(before + 1), which holds only when nothing is open above the lowest free descriptor. A parent that leaves a descriptor open just above it (a lock on fd 9 in the combined test) makes the test fail with no fault in the library.

• The test opens the descriptors it needs and checks those, or closes everything above stdio first, so it does not depend on what the parent left open.
• A case that runs it with a stray descriptor open (e.g. 9) and passes.