test.py: remove use of `lit.util.mkdir_p` (#292)

The helper function `lit.util.mkdir_p` has been removed in
https://github.com/llvm/llvm-project/pull/163948 .

Instead of that, call `pathlib` functions directly.
diff --git a/litsupport/test.py b/litsupport/test.py
index 8e48bd5..1ea37ed 100644
--- a/litsupport/test.py
+++ b/litsupport/test.py
@@ -4,13 +4,13 @@
 """
 import lit
 import lit.TestRunner
-import lit.util
 import lit.formats
 import litsupport.modules
 import litsupport.modules.hash
 import litsupport.testfile
 import litsupport.testplan
 import os
+import pathlib
 
 
 # The ResultCode constructor has been changed recently in lit.  An additional parameter has ben added, which
@@ -38,7 +38,7 @@
 
         # Parse .test file and initialize context
         tmpDir, tmpBase = lit.TestRunner.getTempPaths(test)
-        lit.util.mkdir_p(os.path.dirname(tmpBase))
+        pathlib.Path(tmpBase).parent.mkdir(parents=True, exist_ok=True)
         context = litsupport.testplan.TestContext(test, litConfig, tmpDir, tmpBase)
         litsupport.testfile.parse(context, test.getSourcePath())
         plan = litsupport.testplan.TestPlan()