Flaky test: TestApplicationTest.testStreamingResponse race condition on mingwX64

## Root cause

The test assumes strict alternating message ordering between server-side writes and client-side reads via a Channel. This ordering is not guaranteed because the server coroutine and client coroutine may be scheduled differently on different platforms.

Error on mingwX64:
```
kotlin.AssertionError: Expected , actual <[Client] Test 1>.
```

Also seen on JVM on the `release/3.x` base branch (build 412674).

## Suggested fix

The test design relies on timing-dependent interleaving. Consider using explicit synchronization (e.g., a second channel or barrier) to enforce the expected ordering, or restructure the test to not depend on strict interleaving.

File: `ktor-server/ktor-server-test-host/common/test/TestApplicationTest.kt`

Observed in PR #5465 and on `release/3.x` base branch.

Start with ktor-server/ktor-server-test-host/common/test/TestApplicationTest.kt and inspect TestApplicationTest.testStreamingResponse, focusing on the Channel-based server writes and client reads. Reproduce or run the test on mingwX64 or JVM, then verify it no longer depends on timing-specific message interleaving and passes consistently.

Written by the indexing model from the issue text.