Adding PYTHONSHOWALLOCCOUNT looks rather like a new feature to me. It is on to Bejaming to make a decision about this.

The difference between an environment variable and a command line option is that the former is inherited by children, and the latter is not (this can be considered a drawback or an advantage).

COUNT_ALLOCS is not the only option that spoils Python output. Py_TRACE_REFS adds even more noise to stderr, and this is a default option in debug build. How many tests are still left broken if just change the COUNT_ALLOCS output to stderr instead of stdout?