RFR: 8287872: Disable concurrent execution of hotspot docker tests

[Ioi Lam]

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.

-------------

Commit messages:
 - 8287872: Disable concurrent execution of hotspot docker tests

Changes: https://git.openjdk.java.net/jdk/pull/9047/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=9047&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8287872
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/9047.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/9047/head:pull/9047

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