internal/transport: increase test timeout locally in TestAccountCheckWindowSizeWithLargeWindow (#9182)

Fixes #9061 

## Description
This PR addresses flaky timeout failures in the flow control
verification test (`TestAccountCheckWindowSizeWithLargeWindow`).

### Root Cause
The `testFlowControlAccountCheck` helper processes 5 concurrent streams
with 5 pingpong cycles of 1 MB messages (total 25 MB payload transfer).
On slow or resource-constrained environments, this concurrent network
overhead can exceed the hardcoded 10-second `defaultTestTimeout`,
causing context timeouts, connection drops, and test failures.

### Fix
Instead of modifying the package-wide `defaultTestTimeout` (which could
delay detection of genuine deadlock failures in keepalive tests), this
change overrides the timeout locally:
* Added helper functions `setUpServerOnlyWithTimeout` and
`setUpWithOptionsAndTimeout` to support custom deadlines.
* Overrode the timeout to `30 * time.Second` locally within
`testFlowControlAccountCheck` for both connection setups and stream
contexts.

### Verification
* **Reproduction**: Artificially sleeping 3s per pingpong (requiring
>15s total) with the original 10s timeout reproduced the identical
timeout failures.
* **Fix**: Retaining the 3s delay with the new 30s timeout allowed the
test to pass successfully in 15.10s. All other package tests pass
successfully.

RELEASE NOTES: none