Freeze the structured header tests to a particular commit

Since the spec is still evolving quite a bit and we are
following draft numbers (and not the editor's draft) we
should use the test suite from that draft.

Makefile

@@ -25,7 +25,7 @@ TEST_DEPS = $(if $(CI_ERLANG_MK),ci.erlang.mk) base32 horse proper jsx \
 dep_base32 = git https://github.com/dnsimple/base32_erlang master
 dep_horse = git https://github.com/ninenines/horse.git master
 dep_jsx = git https://github.com/talentdeficit/jsx v2.10.0
-dep_structured-header-tests = git https://github.com/httpwg/structured-header-tests master
+dep_structured-header-tests = git https://github.com/httpwg/structured-header-tests e614583397e7f65e0082c0fff3929f32a298b9f2
 dep_uritemplate-tests = git https://github.com/uri-templates/uritemplate-test master
 
 # CI configuration.