Tests hang with "... is locked, waiting"

[JJ]

If I don't delete the directory after every run, t/unicodable.t hangs writing repeatedly

/tmp/whateverable/rakudo-moar/c80ea2f28939875586d326ca1c63db2cdf7b8c8d is locked. Waiting…

after

ok 57 - \U lookup by code

I do have the "mesozoic" zstd, actually the one built from source.

zstd --version 
*** zstd command line interface 64-bits v1.3.4, by Yann Collet ***

Also rakudo 2018.04.

(Also this happened only when I changed the code to be detected early). It's working now after I put the original Whateverable back. It probably means that zstd is working in some unexpected places, or that some test would be needed to check that it's correct and that it's working)