project tests: fine-tune the check for whether we are in CI

Follow-up on commit 4274e0f42a. We want to
allow tests to be skipped freely in third-party environments, so this
should check the jobname, not whether $CI exists.

We will anyways raise an error when trying to run the project tests, if
$CI is set but no jobname is set.

run_project_tests.py

@@ -973,7 +973,7 @@ def have_java() -> bool:
 
 def skip_dont_care(t: TestDef) -> bool:
     # Everything is optional when not running on CI
-    if not under_ci:
+    if ci_jobname is None:
         return True
 
     # Non-frameworks test are allowed to determine their own skipping under CI (currently)