RFR: JDK-8279486: jdk/javadoc/tool/CheckManPageOptions.java depends on source hierarchy

[Jonathan Gibbons]

Please review a small test update to have this test pass if the main`src/` directory cannot be found. (Yes, apparently that happens in some testing scenarios.)

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Commit messages:
 - JDK-8279486: jdk/javadoc/tool/CheckManPageOptions.java depends on source hierarchy

Changes: https://git.openjdk.java.net/jdk/pull/7240/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7240&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8279486
  Stats: 8 lines in 1 file changed: 6 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7240.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7240/head:pull/7240

PR: https://git.openjdk.java.net/jdk/pull/7240