Tests that require PRNG are by default seeded from `/dev/urandom`.
However, the user should be able to override with a given seed
to reproduce a failing test.
Instead of messing with command-line arguments, a cleaner approach
is to fetch the seed from an environment variable.
- If the utility library is not built, link to the existing library
when building helper programs for filesystem tests.
- Refactor PRNG seeding in tests.
- Move common defs and helper functions to check_util.h/check_utils.c.
- Add a prepare script argument for ATX_CHECK_FS* macros.
- ASSERT_EXPR_INT(): Use long int as expression result type.
- Other misc updates.
