Follow up on minimal target change to 1.8

Open TODO's from https://github.com/eclipse-jdt/eclipse.jdt.core/pull/2551

Created issues:

- [x] Disabled tests summary ticket: https://github.com/eclipse-jdt/eclipse.jdt.core/issues/2758
- [x] https://github.com/eclipse-jdt/eclipse.jdt.core/issues/2580
- [x] https://github.com/eclipse-jdt/eclipse.jdt.core/issues/2754
- [x] https://github.com/eclipse-jdt/eclipse.jdt.core/issues/2755
- [x] https://github.com/eclipse-jdt/eclipse.jdt.core/issues/2756
- [ ] https://github.com/eclipse-jdt/eclipse.jdt.core/issues/2757
- [x] https://github.com/eclipse-jdt/eclipse.jdt.core/issues/2759
- [x] https://github.com/eclipse-jdt/eclipse.jdt.core/issues/2760
- [x] https://github.com/eclipse-jdt/eclipse.jdt.core/issues/2766

Following questions wait for answering by others:
- [ ] [Stephan] Should `this.currentProject.setOption(JavaCore.COMPILER_PB_RAW_TYPE_REFERENCE, JavaCore.IGNORE);` be used in `ConverterTestSetup.setUpJavaProject(String, String)` ?
- [ ] [Stephan]: change on `BatchCompilerTest.testBridgeMethodRetention` This change probably conflicts with the purpose of this test, @srikanth-sankaran ?
- [ ] [Stephan] Change in `ASTConverterTestAST8_2.test0612()` caught me by surprise so I investigated:
* the test implies to test AST at JLS8
* using source from project `Converter`, `Converter15`, however, selects the AST level from the project.
@srikanth-sankaran , @jarthana does it make any sense have such tests here? Shouldn't tests always sit in a test class corresponding to the used `Converter*` project, e.g., `ASTConverterAST3Test`?

Start by reviewing pull request 2551 and the remaining issue 2757, then inspect ConverterTestSetup.setUpJavaProject(String, String), BatchCompilerTest.testBridgeMethodRetention, and ASTConverterTestAST8_2.test0612(). Resolve the listed questions about compiler options, bridge-method retention, and AST converter test placement; done means the outstanding TODOs have clear decisions and corresponding test changes.

Written by the indexing model from the issue text.