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