Fix LLVM test to use %python instead of python

This uses lit substitution, which fixes running this test on
some environment where 'python' isn't in the path.

llvm/utils/lit/tests/lit.cfg

@@ -39,6 +39,7 @@ lit_path = os.path.abspath(lit_path)
 
 # Required because some tests import the lit module
 if llvm_config:
+    llvm_config.use_default_substitutions()
     llvm_config.with_environment("PYTHONPATH", lit_path, append_path=True)
 else:
     config.environment["PYTHONPATH"] = lit_path

llvm/utils/lit/tests/windows-pools.py

@@ -1,7 +1,7 @@
 # Create a directory with 20 files and check the number of pools and workers per pool that lit will use.
 
 # RUN: rm -Rf %t.dir && mkdir -p %t.dir
-# RUN: python -c "for i in range(20): open(rf'%t.dir/file{i}.txt', 'w').write('RUN:')"
+# RUN: %python -c "for i in range(20): open(rf'%t.dir/file{i}.txt', 'w').write('RUN:')"
 
 # RUN:  echo "import lit.formats" > %t.dir/lit.cfg
 # RUN:  echo "config.name = \"top-level-suite\"" >> %t.dir/lit.cfg