[UBSan] Test churn: use the approach from r244839 and r245962 in UBSan lit tests.

Introduce %env_ubsan_opts= substitution instead of specifying
UBSAN_OPTIONS manually in the RUN-lines. This will come in handy
once we introduce some default UBSAN_OPTIONS for the whole testsuite
(for instance, make abort_on_error common option).

llvm-svn: 245967