These can be set in buildenv.pl or a "set" command within a Windows
terminal.  The MSVC script vcregress.pl parses the values available in
the environment to build the resulting prove commands, and the parsing
of PROVE_TESTS is able to handle name patterns in the same way as other
platforms.

Not specifying those environment values makes vcregress.pl fall back to
the previous default, with no extra flags for the prove command, and all
the tests run within t/.

Author: Michael Paquier
Reviewed-by: Juan José Santamaría Flecha, Julien Rouhaud
Discussion: https://postgr.es/m/YD9GigwHoL6lFY2y@paquier.xyz