Access to compiler internal APIs will fail starting with JDK 16

[eamonnmcmanus]

JDK 16 will change the default for the --illegal-access flag from permit to deny. This means that, by default, compile-testing tests will fail with messages like this:

class com.google.testing.compile.Parser (in unnamed module @0x4114d843) cannot access class
com.sun.tools.javac.api.JavacTool (in module jdk.compiler) because module jdk.compiler does not export
com.sun.tools.javac.api to unnamed module @0x4114d843

That can be worked around either by explicitly specifying, on the command line of the JVM running the tests, either --illegal-access=permit or --add-opens=jdk.compiler/com.sun.tools.javac.api=ALL-UNNAMED. We should document that, and perhaps later find a way to make it unnecessary.

[ZacSweers]

Just mentioning that the --illegal-access flag was removed in JDK 17, so any solution for this would likely need to use --add-opens

[h908714124]

Consider deprecating methods containsElementsIn and hasSourceEquivalentTo in class JavaFileObjectSubject. These use Parser internally, which depends on forbidden Sun APIs. With some effort, it should be possible to rewrite affected tests and use method generatedSourceFile in class Compilation instead. This is the future-proof way to compare the generated source with expectations. See this commit for example.