Add missing cleanup in test function

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/tests/suites/test_suite_entropy.function b/tests/suites/test_suite_entropy.function
index d9ea441..2a7b0de 100644
--- a/tests/suites/test_suite_entropy.function
+++ b/tests/suites/test_suite_entropy.function
@@ -217,6 +217,9 @@
 
     for( j = len; j < sizeof( buf ); j++ )
         TEST_ASSERT( acc[j] == 0 );
+
+exit:
+    mbedtls_entropy_free( &ctx );
 }
 /* END_CASE */