Port structure tests to xUnit

I wanted to test whether redmine bug#52 was still there so I copied structure tests from MbUnit suite to xUnit. Looks like that bug was fixed since then, but there were some other bugs in TinySimulator regarding floating point simulation (hopefully those are fixed by these commits too).
Also note I1/I2 tests are disabled because of test failures; I traced the issue down to SByte.Equals/Short.Equals functions which misbehave because of error in CIL.Ceq/IR.IntegerCompare (either one of operands is being promoted to Int32 where it shouldn't, or other doesn't where it should - I'm not sure which way it should be).