[render_gl] Disable multithreaded tests on Ubuntu 24.04

[jwnimmer-tri]

Towards #21335.

See #21420 for a description of the problem. We're temporarily disabling the tests in order to spin up CI, and will circle back later with changes to re-enable.

Making tests conditional on the platform requires some new skylark:

• The "linux only" gains an "enable condition" which can narrow further.
• Add rcfile definitions for our two linux platforms to opt-in.
• Allow setting a test timeout on a linux-only test, which requires switching our stub file from h to cc to avoid confusing the private headers helper macro with an unknown kwarg.

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This change is 

[jwnimmer-tri]

@drake-jenkins-bot linux-noble-unprovisioned-gcc-bazel-experimental-release please

[jwnimmer-tri]

+@SeanCurtis-TRI for both reviews, please (load balancing).