test: make blob desiredSize assertion robust

[trivikr]

Fixes flaky parallel/test-blob failures where
ReadableByteStreamController.desiredSize could be either 0 or -5
after the first blob stream read.

The test checks from a setTimeout(0) callback, while Blob.stream()
continues pulling data via setImmediate(). Depending on event loop
ordering, the next 5-byte "hello" chunk may already be queued by the
time the assertion runs.

This updates the assertion to accept both valid intermediate states and
documents the scheduling dependency.

Refs: https://github.com/nodejs/reliability/issues?q=sort%3Aupdated-desc%20%22parallel%2Ftest-blob%22

Example

not ok 797 parallel/test-blob
  ---
  duration_ms: 252.40200
  severity: fail
  exitcode: 1
  stack: |-
    node:internal/assert/utils:146
      throw error;
      ^
    
    AssertionError [ERR_ASSERTION]: Expected values to be strictly equal:
    
    -5 !== 0
    
        at Timeout.<anonymous> (/home/iojs/build/workspace/node/test/parallel/test-blob.js:358:12)
        at Timeout._onTimeout (/home/iojs/build/workspace/node/test/common/index.js:510:15)
        at listOnTimeout (node:internal/timers:605:17)
        at process.processTimers (node:internal/timers:541:7) {
      generatedMessage: true,
      code: 'ERR_ASSERTION',
      actual: -5,
      expected: 0,
      operator: 'strictEqual',
      diff: 'simple'
    }

---

Assisted-by: openai:gpt-5.5