t7400: test of UTF-8 submodule names pass under Mac OS

submodules with names using UTF-8 need core.precomposeunicode true
under Mac OS X, set it in the test case.

Improve the portability:

  - Not all shells on all OS may understand literal UTF-8 strings.
  - Use a help variable filled by printf, as we do it in e.g. t0050.

"strange names" can be called UTF-8, rephrase the heading.

While at it, unbreak &&-chain in the test, and use test_config.

Signed-off-by: Torsten Bögershausen <tboegi@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>