RFR: Ensure vthread has connected to server

[Miao Zheng]

I have run the test case of InterruptHttp.java in fast debug version of jdk with options:
-XX:+UseG1GC  -XX:+ScavengeALot -XX:ScavengeALotInterval=200 -XX:-GCALotAtAllSafepoints

And the test case failed, The output is "Execution failed: `main' threw exception: java.lang.RuntimeException: Expected 1 HTTP connection"

The reason is Thread.sleep(1000) can not ensure that vthread has connected to server when using some options and fastdebug version of jdk.

So, I fix this to ensure vthread has connected to server.

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Commit messages:
 - Ensure vthread has connected to server

Changes: https://git.openjdk.java.net/loom/pull/60/files
 Webrev: https://webrevs.openjdk.java.net/?repo=loom&pr=60&range=00
  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/loom/pull/60.diff
  Fetch: git fetch https://git.openjdk.java.net/loom pull/60/head:pull/60

PR: https://git.openjdk.java.net/loom/pull/60