Add some basic bls_modular_inverse() tests

tests/core/pyspec/eth2spec/test/deneb/unittests/polynomial_commitments/test_polynomial_commitments.py

@@ -215,6 +215,27 @@ def test_verify_blob_kzg_proof_incorrect_proof(spec):
     assert not spec.verify_blob_kzg_proof(blob, commitment, proof)
 
 
+@with_deneb_and_later
+@spec_test
+@single_phase
+def test_bls_modular_inverse(spec):
+    """
+    Verify computation of multiplicative inverse
+    """
+    rng = random.Random(5566)
+
+    # Should fail for x == 0
+    expect_assertion_error(lambda: spec.bls_modular_inverse(0))
+
+    # Test a trivial inversion
+    assert 1 = int(spec.bls_modular_inverse(1))
+
+    # Test a random inversion
+    r = rng.randint(0, spec.BLS_MODULUS - 1)
+    r_inv = int(spec.bls_modular_inverse(r))
+    assert r * r_inv % BLS_MODULUS == 1
+
+
 @with_deneb_and_later
 @spec_test
 @single_phase