RFR: JDK-8314975: JavadocTester should set source path if not specified

[Jonathan Gibbons]

Please review a test-only fix, for `JavadocTester`, to set the source path to a sensible value if none specified explicitly, to override the tool default to use the class path.

The underlying issue is that TestNG is on the class path, and contains a file `Version.java` in the root directory -- i.e. the unnamed package. This can interfere with various tests that use classes in the unnamed package.

The fix is to set the source path to `testSrc`, so that `testng.jar` is never on the source path.

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Commit messages:
 - JDK-8314975: JavadocTester should set source path if not specified

Changes: https://git.openjdk.org/jdk/pull/15424/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15424&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8314975
  Stats: 12 lines in 1 file changed: 12 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/15424.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15424/head:pull/15424

PR: https://git.openjdk.org/jdk/pull/15424