test: align csp test with pathlib rooting pattern from existing suite

Use Path(__file__).resolve().parents[1] so the test survives being run
from a non-repo-root cwd, matching test_issue1112_csp_google_fonts.py.

Absorbed from PR #1852 follow-up commit 1448f42 by @ChaseFlorell.

Co-authored-by: Chase Florell <ChaseFlorell@users.noreply.github.com>

tests/test_issue1850_csp_connect_src_jsdelivr.py

@@ -6,11 +6,13 @@ jsDelivr and are fetched via connect (not script load), so connect-src must
 include cdn.jsdelivr.net or browsers block the fetch and emit CSP violations.
 """
 import re
+from pathlib import Path
+
+_HELPERS_PY = Path(__file__).resolve().parents[1] / "api/helpers.py"
 
 
 def _helpers_src() -> str:
-    with open("api/helpers.py") as f:
-        return f.read()
+    return _HELPERS_PY.read_text()
 
 
 class TestCSPConnectSrcJsdelivr: