RFR: 8287872: Disable concurrent execution of hotspot docker tests

[Calvin Cheung]

On Mon, 6 Jun 2022 22:52:41 GMT, Ioi Lam <iklam at openjdk.org> wrote:

> Please review this quick fix.
> 
> Before the cause of [JDK-8287744](https://bugs.openjdk.org/browse/JDK-8287744) is known, let's disable concurrent execution of hotspot docker tests to avoid failures in CI.

Looks good.

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Marked as reviewed by ccheung (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/9047