Make it easier to run verification test locally (#100)

* Make it easier to run verification test locally

For #99

If you're only testing verification use-cases locally, you don't need
the OIDC JWT.

Signed-off-by: Zach Steindler <steiza@github.com>

* Nicer error message for not providing arguments

```
sigstore-conformance$ pytest test --entrypoint /Users/steiza/code/steiza/sigstore-conformance/sigstore-python-conformance
Please specify one of '--github-token' or '--skip-signing'
sigstore-conformance$ echo $?
3
```

Signed-off-by: Zach Steindler <steiza@github.com>

---------

Signed-off-by: Zach Steindler <steiza@github.com>

README.md

@@ -62,7 +62,13 @@ invoke the client.
 Running the conformance suite locally,
 
 ```sh
-(env) $ pytest --entrypoint=SIGSTORE_CLIENT --identity-token=$(gh auth token)
+(env) $ pytest test --entrypoint=SIGSTORE_CLIENT --identity-token=$(gh auth token)
+```
+
+Or if you are only checking verification use cases,
+
+```sh
+(env) $ pytest test --skip-signing --entrypoint=SIGSTORE_CLIENT
 ```
 
 Using the [`gh` CLI](https://cli.github.com/) and noting SIGSTORE_CLIENT is the absolute path to a client implementing the [CLI specification](https://github.com/sigstore/sigstore-conformance/blob/main/docs/cli_protocol.md).

test/conftest.py

@@ -36,6 +36,10 @@ class OidcTokenError(Exception):
     pass
 
 
+class ConfigError(Exception):
+    pass
+
+
 def pytest_addoption(parser) -> None:
     """
     Add the `--entrypoint`, `--github-token`, and `--skip-signing` flags to
@@ -52,7 +56,6 @@ def pytest_addoption(parser) -> None:
         "--github-token",
         action="store",
         help="the GitHub token to supply to the Sigstore client under test",
-        required=True,
         type=str,
     )
     parser.addoption(
@@ -68,11 +71,25 @@ def pytest_runtest_setup(item):
 
 
 def pytest_configure(config):
+    if not config.getoption("--github-token") and not config.getoption("--skip-signing"):
+        raise ConfigError("Please specify one of '--github-token' or '--skip-signing'")
+
     config.addinivalue_line("markers", "signing: mark test as requiring signing functionality")
 
 
+def pytest_internalerror(excrepr, excinfo):
+    if excinfo.type == ConfigError:
+        print(excinfo.value)
+        return True
+
+    return False
+
+
 @pytest.fixture
 def identity_token(pytestconfig) -> str:
+    if pytestconfig.getoption("--skip-signing"):
+        return ""
+
     gh_token = pytestconfig.getoption("--github-token")
     session = requests.Session()
     headers = {