test: Skip building the ubsan and openmp tests for arm64ec

Also unset RUN for arm64ec, instead of accidentally inheriting the
earlier value.

test/Makefile

@@ -80,7 +80,9 @@ TESTS_FORTIFY = bufferoverflow crt-test
 ifneq ($(HAVE_ASAN),)
     TESTS_ASAN = stacksmash
 endif
-TESTS_UBSAN = ubsan
+ifneq ($(HAVE_UBSAN),)
+    TESTS_UBSAN = ubsan
+endif
 ifneq ($(HAVE_OPENMP),)
     TESTS_OMP = hello-omp
 endif