ci: test with Java 11

[AirQuick]

Summary

This pull request adds Java 11 to the test matrix of Azure Pipelines and Travis CI.
AppVeyor is left intact, because I suppose Java 11 is not considered as the mainstream support of XSpec.

Why

Although XSpec itself does not depend on a specific Java version, testing with Java 11 is necessary to catch derivative issues like #633 (and consequently nkutsche/xspec-maven-plugin#1).

What has changed

• Azure Pipelines

  • Move the main settings to a template (test/ci/azure-pipelines_template.yml) and parameterize the Java version
  • Create a Java version switcher (test/ci/switch-java.cmd) and call it from the template for switching to Java 11
  • Perform Java compilation only on Java 8 using condition

• Travis CI

  • Add openjdk11 to jdk
  • Externalize Java compilation command line into test/ci/compile-java.sh and perform compilation only on Java 8

How I tested it

• I inspected the console logs on Azure and Travis, and verified these

  • java -version output
  • A Java 11 warning ("Warning: Nashorn engine is planned to be removed from a future JDK release") is observed in Java 11 jobs

• Bats test failure

  • Azure
  • Travis

• XSpec test failure

  • Azure
  • Travis

• End-to-end test failure

  • Azure
  • Travis

• .class file verification failure

  • Azure
  • Travis

Further work

Update Wiki to mention Java 11.

[AirQuick]

Maybe a bit too quick, but I'm merging this, for

• Test failures are demonstrated
• No change in XSpec core code
• We'll need this test duality eventually
• Broadening the test range is generally a good thing
• Some other pull requests depend on this

A drawback is we now exceeds the max concurrent jobs of Travis CI.

[AirQuick]

@xspec/xspec

Further work

Update Wiki to mention Java 11.

Done.
I added (recommended) to Java 8 and Saxon 9.9, just because that's where our CI executes the largest test set.