Previous commits aimed to disable tests when running on Jenkins and/or
GitHub actions. However the conditions were too aggressive in some
cases, causing tests to be skipped even when run locally by devs.
This change aims to improve the enablements so that tests are only
skipped on CI.

`isRunningOnContinousIntegration` was unused, hence removed as it is
related to this PR.

Part of https://github.com/eclipse-platform/eclipse.platform.swt/issues/2714