RFR: 7468: Fix jdp test coverage

[Alex Macdonald]

This PR addresses JMC-7468 [[0]](https://bugs.openjdk.java.net/browse/JMC-7468), in which the jdp test coverage is not added to the core coverage report.

The one-liner fix here is to make sure the argLine used by Jacoco is not overwritten by the surefire-plugin in the jdp.test pom.

Before (0%):
![2021-12-03-141742_1091x251_scrot](https://user-images.githubusercontent.com/10425301/144663611-4424aae8-947c-4006-a26d-335ab325bfef.png)

After (60%):
![2021-12-03-144244_1092x258_scrot](https://user-images.githubusercontent.com/10425301/144663625-5fc72bc8-c243-4f86-b00a-6f9d6a8cc3e7.png)

[0] https://bugs.openjdk.java.net/browse/JMC-7468

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Commit messages:
 - 7468: Fix jdp test coverage

Changes: https://git.openjdk.java.net/jmc/pull/340/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=340&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-7468
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jmc/pull/340.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/340/head:pull/340

PR: https://git.openjdk.java.net/jmc/pull/340