RFR: 7167356: (javac) investigate failing tests in JavacParserTest

[Archie L. Cobbs]

This bug, created back in 2012, notes that three of the unit `@Test`s in `parser/JavacParserTest.java` are disabled because the functionality they verify was not yet implemented correctly at that time.

That was a long time ago, and since then the functionality has been fixed and the tests are now working properly, so they can be enabled at long last.

The third test also needs to be adjusted in order to succeed. It was (for some reason) expecting that an `enum` constant's source position would be reported as offset -1, instead of its actual source position (in this case, 23).

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Commit messages:
 - Correct and enable unit tests that are now working properly.

Changes: https://git.openjdk.org/jdk/pull/12444/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12444&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-7167356
  Stats: 8 lines in 1 file changed: 2 ins; 4 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/12444.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12444/head:pull/12444

PR: https://git.openjdk.org/jdk/pull/12444