RFR: 8281705: SourceLauncherTest.testSystemProperty isn't being run

[Jaikiran Pai]

Can I please get a review of this test-only change which fixes the issue noted in https://bugs.openjdk.java.net/browse/JDK-8281705? 

The change in this commit adds the `@Test` annotation on the test method to make sure it gets run. Before this change, the test run logs showed that `28 tests` were being run and now after this change, the logs report `29 tests` and I can see `testSystemProperty` in the test run logs.

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Commit messages:
 - 8281705: SourceLauncherTest.testSystemProperty isn't being run

Changes: https://git.openjdk.java.net/jdk/pull/7456/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7456&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8281705
  Stats: 3 lines in 1 file changed: 1 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7456.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7456/head:pull/7456

PR: https://git.openjdk.java.net/jdk/pull/7456