RFR: 7446: add Maven profile for enabling test coverage

[Alex Macdonald]

This PR addresses JMC-7446 [[0]](https://bugs.openjdk.java.net/browse/JMC-7446), in which it would be nice to have a Maven profile for enabling test coverage.

At the moment, test coverage is enabled by default with no way of disabling it. This PR adds a Maven profile (in a similar vein to how uitests are run) that allows coverage reports to be enabled or disabled as desired. This profile is included in both the core and application poms.

Usage: `mvn clean install -P coverage`, or `mvn clean verify -P coverage -P uitests`, etc.

[0] https://bugs.openjdk.java.net/browse/JMC-7446

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Commit messages:
 - 7446: add Maven profile for enabling test coverage

Changes: https://git.openjdk.java.net/jmc/pull/336/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=336&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-7446
  Stats: 218 lines in 7 files changed: 129 ins; 78 del; 11 mod
  Patch: https://git.openjdk.java.net/jmc/pull/336.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/336/head:pull/336

PR: https://git.openjdk.java.net/jmc/pull/336