[Test]: Remove redundant modifier tests

[nicolaassolini-qntm]

Tests in the test_files/modifier_examples are a bit redundant. Once all the main issues are closed, we should keep only a few significant tests

[nicolaassolini-qntm]

We may consider moving some of these tests directly to Guppy

[nicolaassolini-qntm]

Also test files may contain a useless sys.path.append(str(Path(__file__).resolve().parents[1])) statement