tests: test-backend-ops -j <N> to run tests in parallel (#23637)

tests: test-backend-ops -j <N> to run tests in parallel (#23637)

Create a pool of N threads that grab a chunk of up to 100 tests at a time to
iterate through. The number of tests at a time decreases as fewer remain.

Each thread uses its own dev and cpu backend, and set_n_threads_fn is not
called on the cpu backend.

Fix some TSAN issues that arose:
- In init_tensor_uniform, don't use static vector of generators.
- Replace gmtime with versions that don't use a global variable.
- Mutex calls to print_test_result.