test: reduce ucx_backend_test transfer volume (#2042)

## Summary

The TSan leg of `nixl-ci-test-sanitizers` was being killed at its 40
minute cap while `ucx_backend_test` was still making steady progress.

In build #649 the stage started at 10:08:23 and was killed at 10:48:22 —
exactly 40:00. `ucx_backend_test` ran from 10:10:16 to the kill (~38
min), printing data-verification output every 8-12s with a largest gap
of ~15s, and was cut off mid-WRITE in the last of its four
configurations. Not a hang, and not a sanitizer finding — it simply does
not fit. Build #632 hit the same wall.

Each configuration moves and verifies `desc_cnt * desc_size` bytes. At
64 descriptors of 1 MB that is 64 MB per configuration, and under
ThreadSanitizer every access on that data path is instrumented.

## Changes

- **`test/unit/plugins/ucx/ucx_backend_test.cpp`** — drop `desc_cnt`
from 64 to 8 in `test_intra_agent_transfer` and
`test_inter_agent_transfer`

## Why this does not reduce coverage

- The configuration matrix (progress thread on/off x handler reuse
on/off) is **unchanged**, so every code path is still exercised.
Dropping configurations would have cost real coverage; reducing the
volume of identical work does not.
- 8 descriptors still exercise the multi-descriptor list handling in
`populateDescs` and the descriptor-list transfer paths.
- `desc_size` deliberately stays at 1 MB, so transfers remain in the
same UCX protocol regime — shrinking the payload instead could cross the
eager/rendezvous threshold and silently change which transport path is
tested.
- ThreadSanitizer reports races from happens-before analysis on the
accesses it observes, not from stress. A race on a given code path is
flagged regardless of how many times that path repeats, so 8 descriptors
detect what 64 would.

Net effect is ~8x less data per configuration, which brings the stage
well inside its budget without touching the timeout.

## Verification

Confirmation is a green `nixl-ci-test-sanitizers` run with the tsan leg
completing inside 40 minutes. The asan_ubsan leg was already passing and
is unaffected in kind, just faster.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
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## Summary by CodeRabbit

* **Tests**
* Reduced the number of transfer descriptors used in UCX transfer tests
while preserving descriptor sizing and overall transfer behavior.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Signed-off-by: NirWolfer <nwolfer@nvidia.com>