t: convert tests to not access symrefs via the filesystem

Some of our tests access symbolic references via the filesystem
directly. While this works with the current files reference backend, it
this will break once we have a second reference backend in our codebase.

Refactor these tests to instead use git-symbolic-ref(1) or our
`ref-store` test tool. The latter is required in some cases where safety
checks of git-symbolic-ref(1) would otherwise reject writing a symbolic
reference.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t9133-git-svn-nested-git-repo.sh

@@ -11,7 +11,7 @@ test_expect_success 'setup repo with a git repo inside it' '
 	(
 		cd s &&
 		git init &&
-		test -f .git/HEAD &&
+		git symbolic-ref HEAD &&
 		> .git/a &&
 		echo a > a &&
 		svn_cmd add .git a &&