chore: add private static variable to skip GCE residency checks

[Hectorhammett]

Problem

When running unit tests on Google-managed machines (such as Cloudtop or GCP VMs), tests in GCECredentialsTest and ApplicationDefaultCredentialsTest that simulate non-GCE environments fail.

Although tests mock HTTP failure responses from the metadata server, GCECredentials::onGce() falls back to checking the host's SMBIOS product name (/sys/class/dmi/id/product_name). Because Cloudtop instances run on Google infrastructure, this file contains Google Compute Engine, causing onGce() to evaluate to true and breaking assertions that expect non-GCE behavior.

Solution

1. Added support for the standard NO_GCE_CHECK environment variable in GCECredentials::onGce() (matching parity with Python google-auth and Java google-auth-library-java). When set, onGce() immediately returns false without pinging or checking SMBIOS.
2. Added a skipGceCheck() helper in tests/bootstrap.php (similar to setHomeEnv()) and updated tests that verify non-GCE behavior.
3. Added a unit test verifying NO_GCE_CHECK behavior.

The tests are now working on my cloudtop.

BEGIN_COMMIT_OVERRIDE
chore(tests): add private static variable to skip GCE residency checks (#682)

END_COMMIT_OVERRIDE

[bshaffer]

My understanding is this issue was introduced with the Linux Residency check. So it would be much more prudent to have the ability to skip there, rather than skip the entire GCE check. Additionally rather than make it a public constant and a public env var, it would be even MORE prudent to make it a private static variable that we can manipulate with Reflection in the tests.

[Hectorhammett]

@bshaffer

Sounds good! I like the way you mention it.

Just for some context I checked that it seems to be a common pattern to have a NO_GCE_CHECK in other languages like here in Java:
https://github.com/googleapis/google-auth-library-java/blob/main/oauth2_http/java/com/google/auth/oauth2/DefaultCredentialsProvider.java#L70

And in Python:
https://github.com/googleapis/google-auth-library-python/blob/main/google/auth/environment_vars.py#L73

So i thought might as well follow this pattern.

But I like your suggestion, updating it!

[bshaffer]

Thank you for fixing this!!