test suite: Annotate memory-hungry tests so that they are not run on low-memory hosts

For example, T16992 requires a large amount of memory. We should add a "test decorator" (I'm not sure the proper name), e.g. skip_if_low_memory, and annotate at least this test to be friendlier to low memory hosts.

ref https://mail.haskell.org/pipermail/ghc-devs/2022-July/020801.html