Good point! This is what happens when you have too many open fronts ;-) Anyhow, I've taken some time to dig into the problem and found the following
less test/lisp/shadowfile-tests.log
Running 8 tests (2021-06-18 08:33:34+0000, selector `(not (or (tag :expensive-test) (tag :unstable)))')
passed 1/8 shadow-test01-sites (0.743613 sec)
passed 2/8 shadow-test02-files (0.007628 sec)
passed 3/8 shadow-test03-expand-cluster-in-file-name (0.023701 sec)
passed 4/8 shadow-test04-contract-file-name (0.007431 sec)
passed 5/8 shadow-test05-file-match (0.023161 sec)
passed 6/8 shadow-test06-literal-groups (0.029197 sec)
passed 7/8 shadow-test07-regexp-groups (0.007492 sec)
Autosave file on local temporary directory, do you want to continue? (yes or no)
test/lisp/shadowfile-tests.log (END)
I went and said 'yes' on the terminal where I was running make and the build process ended.
This happens on commit bc44763b8342ee9e1bf43ffb202c73fee5ba0bf6
Best, /PA