testing using persistent test database instead of 

This addresses:

  1. Tests need to work when MBD_SCRATCH_ is defined but no postgres server binaries

mentioned in #20

It also provides some support for MSWin32 and cygwin, which are good tests for this feature since the server packages for cygwin seem to be completely broken (as far as I can tell) and the way MBD uses unix domain sockets for test/scratch db will never work in MSWin32.