RFR: JDK-8260858: Implementation specific property xsltcIsStandalone for XSLTC Serializer

[Joe Wang]

Adds a property similar to 'isStandalone' in JDK-8249867.

Please note that the test received an auto-format. The material changes were the two tests marked with bug id 8260858 and related data and methods.

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

Commit messages:
 - JDK-8260858: Implementation specific property xsltcIsStandalone for XSLTC Serializer

Changes: https://git.openjdk.java.net/jdk/pull/2559/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2559&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8260858
  Stats: 243 lines in 7 files changed: 167 ins; 4 del; 72 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2559.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2559/head:pull/2559

PR: https://git.openjdk.java.net/jdk/pull/2559