Fix failed test with recent glibc

In recent version of strptime glibc upstream has fixed a discrepancy between
documentation and actual behaviour by skipping over the sequence that would match %Z.
See http://repo.or.cz/w/glibc.git/commitdiff/ddc7e412ab252e7360a4357664beb3b5d9c4f42b

So when %Z is used in format
glibc <= 2.18 returns the GMT as unparsed
glibc >= 2.19 returns empty string

Removing %Z from tested format gives the same behavior with all version.