CL 516860 accidentally changed the randomness
used in TempFile from 32 to 64 bits on 64-bit platforms,
meaning from 10 to 20 decimal bytes.
This is enough to cause problems in a few tests
because it makes temporary directory names just
a little bit longer.
Limit back down to 32 bits of randomness, which is fine,
and add a test to avoid repeating the mistake.