t5703: adjust a test case for the upcoming default branch name

We want to rename the default branch name used by `git init` in the near
future, using `main` as the new name.

In preparation for that, we adjust a test case that wants to rename the
default branch to a different name that however has the same length. We
use `none` as that name because it matches the length of `main`.

As this test case cannot possibly pass until the default branch name is
_actually_ changed, we temporarily guard it behind a special-purpose
prereq, until the test suite is fully converted to use that new default
branch name.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5703-upload-pack-ref-in-want.sh

@@ -383,14 +383,14 @@ test_expect_success 'server is initially behind - ref in want' '
 	test_cmp expected actual
 '
 
-test_expect_success 'server loses a ref - ref in want' '
+test_expect_success PREPARE_FOR_MAIN_BRANCH 'server loses a ref - ref in want' '
 	git -C "$REPO" config uploadpack.allowRefInWant true &&
 	rm -rf local &&
 	cp -r "$LOCAL_PRISTINE" local &&
-	echo "s/master/raster/" >"$HTTPD_ROOT_PATH/one-time-perl" &&
+	echo "s/main/rain/" >"$HTTPD_ROOT_PATH/one-time-perl" &&
 	test_must_fail git -C local fetch 2>err &&
 
-	test_i18ngrep "fatal: remote error: unknown ref refs/heads/raster" err
+	test_i18ngrep "fatal: remote error: unknown ref refs/heads/rain" err
 '
 
 # DO NOT add non-httpd-specific tests here, because the last part of this