[roottest] Make ALICE-18972 test conditional on geom

The test includes the TGeomManager, so it should be only added if ROOT
was built with `geom=ON`.

Author: guitargeek

roottest/cling/parsing/ALICE-18972/CMakeLists.txt

@@ -1,6 +1,7 @@
 ROOTTEST_GENERATE_REFLEX_DICTIONARY(ALICE_18972 mylib.h SELECTION mylib_sel.xml)
 
-ROOTTEST_ADD_TEST(ALICE_18972
-                  MACRO run_ALICE_18972.C
-                  DEPENDS ${GENERATE_REFLEX_TEST})
-
+if(ROOT_geom_FOUND)
+  ROOTTEST_ADD_TEST(ALICE_18972
+                    MACRO run_ALICE_18972.C
+                    DEPENDS ${GENERATE_REFLEX_TEST})
+endif()