gh-150313: Add tests for computed flow control write limits

[azibom]

Adds test coverage for computed write buffer limit behavior in
asyncio._FlowControlMixin.

The new tests verify:

• default computed limits: (16 * 1024, 64 * 1024)
• high computed from low
• low computed from high

These behaviors are implemented in _set_write_buffer_limits()
but were not explicitly covered by tests.

Tests help protect against future regressions in flow control
buffer calculations.

Closes gh-150313.

[bedevere-app]

Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

If this change has little impact on Python users, wait for a maintainer to apply the skip news label instead.

[python-cla-bot]

The following commit authors need to sign the Contributor License Agreement:

• you@example.com

[bedevere-app]

Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

If this change has little impact on Python users, wait for a maintainer to apply the skip news label instead.