Fix setting for default test suite directory

Signed-off-by: Werner Lewis <werner.lewis@arm.com>

scripts/mbedtls_dev/test_generation.py

@@ -138,7 +138,8 @@ class BaseTarget(metaclass=ABCMeta):
 class TestGenerator:
     """Generate test cases and write to data files."""
     def __init__(self, options) -> None:
-        self.test_suite_directory = getattr(options, 'directory', 'tests/suites')
+        self.test_suite_directory = self.get_option(options, 'directory',
+                                                    'tests/suites')
         # Update `targets` with an entry for each child class of BaseTarget.
         # Each entry represents a file generated by the BaseTarget framework,
         # and enables generating the .data files using the CLI.
@@ -147,6 +148,11 @@ class TestGenerator:
             for subclass in BaseTarget.__subclasses__()
         })
 
+    @staticmethod
+    def get_option(options, name: str, default: T) -> T:
+        value = getattr(options, name, None)
+        return default if value is None else value
+
     def filename_for(self, basename: str) -> str:
         """The location of the data file with the specified base name."""
         return posixpath.join(self.test_suite_directory, basename + '.data')