tests/slice-concurrent.c: avoid using rand() from multiple threads

The rand() function is not thread safe, and calling it from multiple threads can result in data corruption (especially with weaker memory models such as Arm). I encountered this data corruption as a buffer overflow while running the tests on an Arm Morello system. It appears that this does not cause issues in most configurations since GLibc uses a global lock and most of the time these tests are run on architectures with stronger memory ordering.

See https://github.com/CTSRD-CHERI/cheribsd/issues/1584 for more details. Split out from !3135 (closed) (not CHERI specific, general fix)