Add support for PROVE_TESTS and PROVE_FLAGS in MSVC scripts

 Add support for PROVE_TESTS and PROVE_FLAGS in MSVC scripts
 
 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