RFR: 8281379: Assign package declarations to all jtreg test cases under gc

[Leo Korinth]

Assign package declarations to all jtreg test cases under directory "gc". At the moment a few test cases are using no explicit package which makes it hard editing the testcase in an IDE. 

hotspot_gc passed.

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Commit messages:
 - 8281379: Assign package declarations to all jtreg test cases under gc

Changes: https://git.openjdk.java.net/jdk/pull/7370/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7370&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8281379
  Stats: 20 lines in 5 files changed: 8 ins; 0 del; 12 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7370.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7370/head:pull/7370

PR: https://git.openjdk.java.net/jdk/pull/7370