http2: verify that a dependency cycle may exist

[jasnell]

Simple test to verify that a priority dependency cycle may exist but won't crash anything.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

http2

[jasnell]

CI: https://ci.nodejs.org/job/node-test-pull-request/12426/

[jasnell]

Landed in 22760f3

[MylesBorins]

This lands cleanly on v8.x but the test fails. Likely blocked by another backport

=== release test-http2-priority-cycle- ===
Path: parallel/test-http2-priority-cycle-
(node:86045) ExperimentalWarning: The http2 module is an experimental API.
/Users/mborins/code/node/v8.x/test/parallel/test-http2-priority-cycle-.js:45
    client.close();
           ^

TypeError: client.close is not a function
    at Countdown (/Users/mborins/code/node/v8.x/test/parallel/test-http2-priority-cycle-.js:45:12)
    at Countdown.dec (/Users/mborins/code/node/v8.x/test/common/countdown.js:19:22)
    at ClientHttp2Stream.req.on (/Users/mborins/code/node/v8.x/test/parallel/test-http2-priority-cycle-.js:67:37)
    at emitOne (events.js:116:13)
    at ClientHttp2Stream.emit (events.js:211:7)
    at emit (internal/http2/core.js:144:8)
    at _combinedTickCallback (internal/process/next_tick.js:141:11)
    at process._tickCallback (internal/process/next_tick.js:180:9)