tests/core: Really pick C.UTF-8 locale

tests/core: Really pick C.UTF-8 locale

The case-ignoring regex `^(C|en_US)` will match any locale that starts
with `c`. On my system this is `ca_AD.utf8`, which breaks the test
suite. Instead, use a single regex that includes the joining `.` rather
than 2 separate regexes. This also changes `head` to use the `-n`
option, which has been preferred for at least 10 years in the coreutils
version and is supported by busybox as well.