Added extra tests to please codecov

tests/estimation_tests/test_genetic_programming_regression_fitter.py

@@ -1,5 +1,6 @@
 import unittest
 import pandas as pd
+from operator import sub
 
 from causal_testing.estimation.genetic_programming_regression_fitter import GP
 
@@ -8,3 +9,21 @@ class TestGP(unittest.TestCase):
     def test_init_invalid_fun_name(self):
         with self.assertRaises(ValueError):
             GP(df=pd.DataFrame(), features=[], outcome="", max_order=2, sympy_conversions={"power_1": ""})
+
+    def test_simplify_string(self):
+        gp = GP(
+            df=None,
+            features=["x1"],
+            outcome=None,
+            max_order=1,
+        )
+        self.assertEquals(str(gp.simplify("power_1(x1)")), "x1")
+
+    def test_fitness_string(self):
+        gp = GP(
+            df=pd.DataFrame({"x1": [1, 2, 3], "outcome": [2, 3, 4]}),
+            features=["x1"],
+            outcome="outcome",
+            max_order=0,
+        )
+        self.assertEquals(gp.fitness("add(x1, 1)"), (0,))