RFR: 8284585: PushPromiseContinuation test fails intermittently in timeout

[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.

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Commit messages:
 - 8284585: PushPromiseContinuation test fails intermittently in timeout

Changes: https://git.openjdk.java.net/jdk/pull/8518/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8518&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8284585
  Stats: 63 lines in 4 files changed: 43 ins; 17 del; 3 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