fix testUpgradeWithUpgradePayloadInlineWithRequestWorks race (#442)
fix testUpgradeWithUpgradePayloadInlineWithRequestWorks race (#442)

Motivation:

testUpgradeWithUpgradePayloadInlineWithRequestWorks has a race condition
where two bytes could be read in one go whereas the expectation was that
they're sent separately.

Modifications:

fix the above race

Result:

- more stable tests
- fixes #434
apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio