add tests for config sanitation

tests/generator/test_config.py

-from os import path
+import pytest
 
-from pystencilssfg import SourceFileGenerator
 from pystencilssfg.config import (
     SfgConfig,
     OutputMode,
     GLOBAL_NAMESPACE,
     CommandLineParameters,
+    SfgConfigException
 )
 
 
@@ -54,6 +54,25 @@ def test_override():
     assert cfg1.clang_format.binary == "bogus"
 
 
+def test_sanitation():
+    cfg = SfgConfig()
+
+    cfg.extensions.header = ".hxx"
+    assert cfg.extensions.header == "hxx"
+
+    cfg.extensions.header = ".cxx"
+    assert cfg.extensions.header == "cxx"
+
+    cfg.clang_format.force = True
+    with pytest.raises(SfgConfigException):
+        cfg.clang_format.skip = True
+
+    cfg.clang_format.force = False
+    cfg.clang_format.skip = True
+    with pytest.raises(SfgConfigException):
+        cfg.clang_format.force = True
+
+
 def test_from_commandline(sample_config_module):
     from argparse import ArgumentParser