RFR: 7503: Fix serializers.test dependency

[Brice Dutheil]

This simple fix properly declares the `flightrecorder.test` dependency as a `test-jar`. This fixes compilation issues in IntelliJ especially.

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Commit messages:
 - Fixes org.openjdk.jmc.flightrecorder.serializers.test dependencies

Changes: https://git.openjdk.java.net/jmc/pull/351/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=351&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-7503
  Stats: 6 lines in 1 file changed: 2 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jmc/pull/351.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/351/head:pull/351

PR: https://git.openjdk.java.net/jmc/pull/351