CI systems typically set the environment variable CI as a de facto standard for easy CI detection. Here are details of several popular CI services:

• GitHub Actions
• Travis-CI
• AppVeyor
• CircleCI
• GitLab
• Bitbucket Pipeline

In general, across programming languages, test frameworks allow behavior changes based on the environment variables set by the CI system.

Python Pytest

Pytest handles conditional tests well. This allows skipping a subset of tests on CI by detecting the CI environment variable:

import os
import pytest

CI = os.environ.get('CI') in ('True', 'true')


@pytest.mark.skipif(CI, reason="not a test for CI")
def test_myfun():
    ...

CMake CTest

CTest can also use CI environment variables to adjust test behavior.

if("$ENV{CI}")
  set(NO_GFX on)
endif()

add_test(NAME menu COMMAND menu)
set_tests_properties(menu PROPERTIES DISABLED $<BOOL:${NO_GFX}>)

Always enclose "$ENV{}" in quotes in case the environment variable is empty or not defined, or the configuration step may syntax error.