Use the path of the current executable by default in soft debugger tests on

Use the path of the current executable by default in soft debugger tests on
Windows to launch the child mono process

The 'mono' shell script generated by the Cygwin make file system doesn't work
when running the soft debugger tests against an MSVC built mono executable.
This patch first tries with the main module of the current process when
running on Windows (and mono is built using MSVC). If it looks like a mono
executable the tests will use it before falling back to the shell script.