[lit] Drop the user-site packages directory from search paths when ru…

…nning tests

Do not add user-site packages directory to the python search path.
This avoids test failures if there's an incompatible lit module installed
inside the user-site packages directory, as it gets prioritized over the lit
from the PYTHONPATH.

llvm/utils/lit/tests/lit.cfg

@@ -40,6 +40,10 @@ if llvm_config:
   llvm_config.with_environment('PYTHONPATH', lit_path, append_path=True)
 else:
   config.environment['PYTHONPATH'] = os.pathsep.join([lit_path])
+# Do not add user-site packages directory to the python search path. This avoids test failures if there's an
+# incompatible lit module installed inside the user-site packages directory, as it gets prioritized over the lit
+# from the PYTHONPATH.
+config.environment['PYTHONNOUSERSITE'] = '1'
 
 # Add llvm and lit tools directories if this config is being loaded indirectly.
 # In this case, we can also expect llvm_config to have been imported correctly.