Added mechanism for very precise self-testing:

Added mechanism for very precise self-testing:
- in selftest mode (-t) interpret comments of the form /* ERROR */ and /* SYNC */
  and validate reported errors with the error markings in a file
- added initial selftest.go file

Also:
- fixed an issue with empty blocks
- generally report better error messages
- added many more tests to the test script (essentially all .go programs which
  have no syntax errors)

R=r
OCL=17426
CL=17426