Add 11 KUnit cases for the RX allocation, submission, completion and
retry paths. Cover each retryable completion followed by ENOMEM/EAGAIN,
batch sizes 1, 8, 9 and 32, skb allocation failure, startup failure
positions, cancellation and shutdown.

Run the actual RX helpers and worker with task-scoped stubs for
allocation and USB submission. Observer references check that the driver
releases its URB and skb references. A delayed-work case checks that
one retry request schedules the submission worker.

Assisted-by: GPT-6 Astra
Signed-off-by: kimwooseok <5mghybrid@khu.ac.kr>
---
 .../wireless/realtek/rtl8xxxu/.kunitconfig    |  16 +
 drivers/net/wireless/realtek/rtl8xxxu/Kconfig |  11 +
 .../net/wireless/realtek/rtl8xxxu/Makefile    |   3 +
 drivers/net/wireless/realtek/rtl8xxxu/core.c  |  66 ++-
 .../net/wireless/realtek/rtl8xxxu/rx-test.c   | 461 ++++++++++++++++++
 .../net/wireless/realtek/rtl8xxxu/rx-test.h   |  27 +
 6 files changed, 569 insertions(+), 15 deletions(-)
 create mode 100644 drivers/net/wireless/realtek/rtl8xxxu/.kunitconfig
 create mode 100644 drivers/net/wireless/realtek/rtl8xxxu/rx-test.c
 create mode 100644 drivers/net/wireless/realtek/rtl8xxxu/rx-test.h