Isolation checks under MSVC

I've committed a bunch of changes both in the Postgres code and the 
buildfarm code to enable running the isolation checks under MSVC. 
There's one hurdle that I haven't overcome: the code tries to call 
"./isolationtester" and Windows barfs on it. I think we need to remove 
that way of doing things, and instead user a full path for such commands 
like we do elsewhere.

For now I have disabled the test on the one MSVC member I had enabled it 
for, to clear the buildfarm error. I had hoped to get this completed 
before I leave for pgcon in the morning, but now it will have to wait a 
week before I am able to test it again, as I won't have a suitable 
machine with me in Ottawa.

cheers

andrew