Having investigated further, the test is wrong, and you should just ignore the failures
These are syntactically illegal setlocale() calls on OpenBSD, and so properly return failure.
(The standard does not dictate a syntax for these, and different OS's are free to choose whatever they want.
The tests use a syntax that is valid on Linux and Darwin.)
On threaded builds, I wrote the code to accept different setlocale() syntaxes, but I didn't want to
perturb (hence possibly break) anything on non-threaded builds, which is all that locales had worked under prior to 5.28.
I will fix blead to skip these tests on unthreaded builds on OpenBSD, but I doubt that
this problem will be considered severe enough to have the fix make it into a 5.28 maintenance release.