I upgraded to Fedora 28 beta over the weekend (Fedora 28 is scheduled 
for release next week) and ran into one test case failure on the 
emacs-26 branch: fns-tests-collate-sort fails (test/src/fns-tests.log is 
attached). Looking into the test, it appears that the test is making 
unportable assumptions about strcoll behavior in non-POSIX locales, an 
assumption that is not true of glibc 2.27 as shipped in Fedora 28. 
Proposed patch attached.