Fix createdb tests for C locale

Fix createdb tests for C locale

If the createdb tests run under the C locale, the database cluster
will be initialized with encoding SQL_ASCII.  With the checks added in
c7db01e325a530ec38ec7ba57cd3ed32e123e33c, this will cause several
ICU-related tests to fail because SQL_ASCII is not supported by ICU.
To work around that, use initdb option -E UTF8 for those tests to get
past that check.