RFR: 8284585: PushPromiseContinuation test fails intermittently in timeout [v3]

[Conor Cleary]

> **Issue**
> A recent fix for [JDK-8263031](https://bugs.openjdk.java.net/browse/JDK-8263031), which addressed the httpclient being unable to properly process Http/2 PushPromise frames followed by continuations caused intermittent failures of the test. This was cause by two seperate problems. 
> 
> - Firstly, on the client side, `Http2Connection.processFrame` did not a check for whether or not a Continuation was to be expected early on in the method resulting in frames other than the expected Continuation Frame being incorrectly processed. In Http/2, when a Continuation Frame is expected, no other type of frame should be processed without a connection error of type PROTOCOL_ERROR being thrown.
> - Secondly, `Http2TestConnection.handlePush` had no guards around response headers and data frames being sent before a continuation is sent which resulted in the intermittent nature of this timeout. 
> 
> **Proposed Solution**
> The test class `OutgoingPushPromise` was modified to contain a list of Continuation Frames as required rather than the Continuations being scheduled to send in the test itself. This prevents the situation of unexpected frames being sent before a Continuation Frame was meant to arrive. 
> 
> In addition to the above, Http2Connection was modified to ensure that a connection error of type PROTOCOL_ERROR is thrown if a frame other than a Continuation arrives at the client unexpectedly.

Conor Cleary has updated the pull request incrementally with one additional commit since the last revision:

  8284585: Added test case to verify fix

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8518/files
  - new: https://git.openjdk.java.net/jdk/pull/8518/files/24e702a8..64fa937f

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8518&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8518&range=01-02

  Stats: 81 lines in 3 files changed: 58 ins; 9 del; 14 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8518.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8518/head:pull/8518

PR: https://git.openjdk.java.net/jdk/pull/8518