t: use user-specified utf-8 locale for testing svn

t: use user-specified utf-8 locale for testing svn

In some test-cases, UTF-8 locale is required. To find such locale,
we're using the first available UTF-8 locale that returned by
"locale -a".

However, the locale(1) utility is unavailable on some systems,
e.g. Linux with musl libc.

However, without "locale -a", we can't guess provided UTF-8 locale.

Add a Makefile knob GIT_TEST_UTF8_LOCALE and activate it for
linux-musl in our CI system.

Rename t/lib-git-svn.sh:prepare_a_utf8_locale to prepare_utf8_locale,
since we no longer prepare the variable named "a_utf8_locale",
but set up a fallback value for GIT_TEST_UTF8_LOCALE instead.
The fallback will be LC_ALL, LANG environment variable,
or the first UTF-8 locale from output of "locale -a", in that order.

Signed-off-by: Đoàn Trần Công Danh <congdanhqx@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>