RFR: 8303922: build-test-lib target is broken

[Eirik Bjorsnos]

The Make target 'build-test-lib-target' is broken in a few ways:

- make/test/BuildTestLib.gmk references the directory $(TEST_LIB_SOURCE_DIR)/sun which does not seem to exist. This can be fixed by removing the reference.
- Some test-lib sources use preview-features which is not enabled by make/test/BuildTestLib.gmk. This is fixed by adding a JAVAC_FLAGS with --enable-preview and also adding 'preview' to DISABLED_WARNINGS
- ASN1Formatter.annotate has a possible lossy conversion from long to int which can be fixed by adding an explicit cast

This PR fixes the above issues.

-------------

Commit messages:
 - Update copyright years
 - Some test-lib sources use preview features, add --enable-preview to JAVAC_FLAGS and add "preview" as a DISABLED_WARNING
 - The directory $(TEST_LIB_SOURCE_DIR)/sun no longer seems to exist, remove it from SRC
 - Fix possible lossy conversion by adding and explicit cast

Changes: https://git.openjdk.org/jdk/pull/12960/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12960&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303922
  Stats: 8 lines in 2 files changed: 2 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/12960.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12960/head:pull/12960

PR: https://git.openjdk.org/jdk/pull/12960