Extend windows detection in testsuite to recognize MSYS target

Currently, the detection recognizes the following `uname -s` strings:

 - `CYGWIN_NT-6.3`
 - `MINGW32_NT-6.3`
 - `MINGW64_NT_6.3`

However, MSYS2 provides an additional target, in which case `uname -s`
returns a string such as `MSYS_NT-6.3`. In all these cases, the system
ought to be recognized as being a `windows` os by the testsuite runner.

See also #9604