skip leaking test in class feature

[iabyn]

This test is known to leak: see GH #20812. Skip it for now so that ASAN smokes don't fail. Start failing again on the next development branch so that the issue isn't forgotten

[leonerd]

LGTM

[jkeenan]

Configuring in blead with these arguments:

$ sh ./Configure -des -Dusedevel -Dcc=clang -Accflags="-g -fno-omit-frame-pointer -fsanitize=address  -fno-common -fsanitize-blacklist=`pwd`/asan_ignore" -Aldflags="-fsanitize=address"

... I then ran the test file:

$ cd t;PERL_DESTRUCT_LEVEL=2 ./perl harness ./lib/croak.t; cd -

... and confirmed the test failure.

I then switched to davem/skip_croak, repeated the process, and confirmed that the failing test is now skipped. Merging.