[Hotfix] Fix FIL gtest

[hcho3]

Fix failing tests in gtest for FIL.

[  FAILED  ] FilTests/TreeliteDenseFloat32FilTest.Import/0, where GetParam() = num_rows = 20000, num_cols = 50, nan_prob = 0.05, depth = 8, num_trees = 50, leaf_prob = 0.05, output = RAW, threshold = 0, threads_per_tree = 1, n_items = 0, blocks_per_sm = 0, algo = 1, seed = 42, tolerance = 0.002, op = <, global_bias = 0, leaf_algo = 0, num_classes = 1, node_categorical_prob = 0, feature_categorical_prob = 0, cat_match_prob = 0.5, max_magnitude_of_matching_cat = 1