RFR: 7447: add serializers and writer modules to core test coverage

[Alex Macdonald]

This PR addresses JMC-7447 [[0]](https://bugs.openjdk.java.net/browse/JMC-7447), in which the core coverage reports are missing the serializers and writer modules. The changes here simply add the missing modules to the coverage dependency list.

Additionally, the jdp.test module is missing the `<scope>test</scope>` attribute, so I've added it in here.

[0] https://bugs.openjdk.java.net/browse/JMC-7447

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Commit messages:
 - 7447: add serializers and writer modules to core test coverage

Changes: https://git.openjdk.java.net/jmc/pull/335/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=335&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-7447
  Stats: 25 lines in 1 file changed: 23 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jmc/pull/335.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/335/head:pull/335

PR: https://git.openjdk.java.net/jmc/pull/335