RFR: Better jdb support for vthreads. Improved jdb vthread testing support. [v3]

[Chris Plummer]

On Tue, 8 Mar 2022 08:08:07 GMT, Serguei Spitsyn <sspitsyn at openjdk.org> wrote:

> It is better to add a comment explaining this.
> Otherwise, it is hard to understand why this conclusion was made.

I added some comments.

> Also, why do you think this is not equal to your conditional assert above?:
> assert !added || eventThread.isVirtual());

Actually that does work, but I think it is easier to read with the "if" statement.

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PR: https://git.openjdk.java.net/loom/pull/97