[jittester] Improve separation of test generation and execution

JitTester is now only capable to run its own tests. In addition to actual test generation, it does test runner work (compilation, interpreted vs compiled executions, etc.). Those are pretty different tasks and I suggest to make a clearer distinction of them - to allow of using JitTester generations in other tools and allow JitTesterRunner, for example, to run tests generated by other tools.
Suggested improvements are:

- Separate individual test generation from java file writing, compiling, executing, etc.;
- Introduce distinct Phases of the generation process (Generation, Compilation, GoldRun and VerificationRun);
- Extract headers generation so that it would be possible to provide other header generators;
- Introduce error tolerance to not get distracted by OOMEs, intrinsics missing in the compiled code, etc.;
- Make it possible to specify time limit for an individual test generation;
- Give better control over temp/workdir creation and cleaning;
- Unify external process running;
- Introduce UTF-8 support in external processes' output and human-readable escaping of it;