test: drop PtEtaPhiM isSpacelike from ROOT xfail list

All 168 parametrizations of test_isSpacelike now match ROOT for the
PtEtaPhiM coordinate system after the coordinate-convention fix, so the
function-level xfail marker is removed to restore a hard PASS signal.

Assisted-by: ClaudeCode:claude-opus-4.8

Author: henryiii

tests/root/conftest.py

@@ -99,7 +99,6 @@
         "test_add",
         "test_eq",
         "test_isLightlike",
-        "test_isSpacelike",
         "test_isTimelike",
         "test_mul",
         "test_neg",