runMain() method that can be used for lazy-loading

[exteon]

@epdenouden I added the runMain() method that serializes running a callback on the main process (and returns the result, which should be serializable). The callback must be a class static method; any references to objects could be statically saved in the main thread by overriding startRunner(). This can be used to do lazy-loading.

[exteon]

@epdenouden I think it is about final, I switched to using PHPUnit's own procedures for marshalling back the test results (so now test statistics are reported back, identical to its own process). The class isolation is implemented properly. Lazy-loading and main thread communication are possible via runMain(), but I haven't added any semantics to it. It needs patching the PHPUnit (method visibilities only) to allow descendant invocation, I'll submit the patch to PHPUnit when I think it's ready. I think it's about final for our purposes.