Add tests for computed write buffer limits in asyncio FlowControlMixin

[azibom]

The _FlowControlMixin._set_write_buffer_limits() method computes default
values when only high or low is provided:

• if low is provided and high is None -> high = 4 * low
• if high is provided and low is None -> low = high // 4
• default values are (16 * 1024, 64 * 1024)

These behaviors are currently not explicitly covered by tests.

This issue proposes adding tests to verify the computed write buffer
limit behavior and protect against future regressions.

Linked PRs

• gh-150315: Add tests for computed flow control write limitsAsyncio flowcontrol tests #150316
• gh-150315: increase test coverage for asyncio._FlowControlMixin.set_write_buffer_limits #150317