Auto merge of #14039 - metajack:windows-wpt, r=jgraham

Fix test-wpt and test-css for Windows.

<!-- Please describe your changes on the following line: -->
In addition to minor changes for Windows, this forces Windows Python to
be used for all Windows builds (instead of using Windows Python only for
pc-windows-msvc builds).

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

<!-- Either: -->
- [ ] There are tests for these changes OR
- [x] These changes do not require tests because testing the tests is too meta for me

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

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This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/14039)
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python/mach_bootstrap.py

@@ -76,9 +76,16 @@ CATEGORIES = {
 }
 
 # Possible names of executables
-PYTHON_NAMES = ["python-2.7", "python2.7", "python2", "python"]
-VIRTUALENV_NAMES = ["virtualenv-2.7", "virtualenv2.7", "virtualenv2", "virtualenv"]
-PIP_NAMES = ["pip-2.7", "pip2.7", "pip2", "pip"]
+# NOTE: Windows Python doesn't provide versioned executables, so we must use
+# the plain names. On MSYS, we still use Windows Python.
+if sys.platform in ['msys', 'win32']:
+    PYTHON_NAMES = ["python"]
+    VIRTUALENV_NAMES = ["virtualenv"]
+    PIP_NAMES = ["pip"]
+else:
+    PYTHON_NAMES = ["python-2.7", "python2.7", "python2", "python"]
+    VIRTUALENV_NAMES = ["virtualenv-2.7", "virtualenv2.7", "virtualenv2", "virtualenv"]
+    PIP_NAMES = ["pip-2.7", "pip2.7", "pip2", "pip"]
 
 
 def _get_exec_path(names, is_valid_path=lambda _path: True):
@@ -197,10 +204,11 @@ def bootstrap(topdir):
 
     # We don't support MinGW Python
     if os.path.join(os.sep, 'mingw64', 'bin') in sys.executable:
-        print('Cannot run mach with MinGW Python.')
-        print('\nPlease rename following files:')
-        print(' /mingw64/bin/python2.exe   -> /mingw64/bin/python2-mingw64.exe')
-        print(' /mingw64/bin/python2.7.exe -> /mingw64/bin/python2.7-mingw64.exe')
+        print('Cannot run mach with MinGW or MSYS Python.')
+        print('\nPlease add the path to Windows Python (usually /c/Python27) to your path.')
+        print('You can do this by appending the line:')
+        print('    export PATH=/c/Python27:$PATH')
+        print('to your ~/.profile.')
         sys.exit(1)
 
     # Ensure we are running Python 2.7+. We put this check here so we generate a

python/servo/command_base.py

@@ -107,7 +107,10 @@ def host_triple():
     elif os_type == "android":
         os_type = "linux-androideabi"
     elif os_type == "windows":
-        os_type = "pc-windows-msvc"
+        if os.getenv("MSYSTEM") is None:
+            os_type = "pc-windows-msvc"
+        else:
+            os_type = "pc-windows-gnu"
     elif os_type.startswith("mingw64_nt-") or os_type.startswith("cygwin_nt-"):
         os_type = "pc-windows-gnu"
     elif os_type == "freebsd":