[libcxx] [test] Quote the python executable in the executor (#68208)

This reapplies the change from
c218c80, which was lost in the
refactoring in 78d649a.

On Windows, the Python interpreter is by default installed in a path
like "C:\Program Files\Python38\python.exe", which requires quoting when
included in a concatenated command string (as opposed to a command list,
where each argument is a separate element).

This doesn't show up in the CI environments as they use Python installed
in a different directory, like C:\Python.

In addition to the python executable, also quote the path to run.py, if
needed.

Author: mstorsjo

libcxx/utils/libcxx/test/params.py

@@ -7,6 +7,7 @@
 # ===----------------------------------------------------------------------===##
 import sys
 import re
+import shlex
 from pathlib import Path
 
 from libcxx.test.dsl import *
@@ -320,7 +321,7 @@ def getStdFlag(cfg, std):
     Parameter(
         name="executor",
         type=str,
-        default=f"{sys.executable} {Path(__file__).resolve().parent.parent.parent / 'run.py'}",
+        default=f"{shlex.quote(sys.executable)} {shlex.quote(str(Path(__file__).resolve().parent.parent.parent / 'run.py'))}",
         help="Custom executor to use instead of the configured default.",
         actions=lambda executor: [AddSubstitution("%{executor}", executor)],
     )