t5551: "GIT_TEST_LONG=Yes make test" is broken

t5551: "GIT_TEST_LONG=Yes make test" is broken

The "test_expect_success 'tag following always works over v0 http'"
test in t5551 fails when it tries to run "git init tags", but this
happens only when EXPENSIVE test is allowed to run.

This is because the step tries to create a repository with "git init
tags" but the EXPENSIVE test that runs way before it creates and
leaves around a temporary file "tags".  Have the EXPENSIVE test
clean it up after itself.

Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>