Reduce the number of test runs for Valgrind tests

This should drastically reduce the runtime of the valgrind tests

test/test_valgrind.py

@@ -34,6 +34,7 @@ def test_valgrind(implementation: pqclean.Implementation, impl_path, test_dir,
                  SCHEME_DIR=os.path.abspath(impl_path),
                  IMPLEMENTATION=implementation.name,
                  DEST_DIR=dest_dir,
+                 NTESTS=1,
                  working_dir=os.path.join(test_dir, 'test'))
     functest_name = './functest_{}_{}'.format(implementation.scheme.name,
                                               implementation.name)