Pin the structured-headers-test repo

Since a draft for a revision was introduced the tests have
started covering additions that we don't currently support.

Makefile

@@ -26,7 +26,7 @@ dep_base32 = git https://github.com/dnsimple/base32_erlang main
 dep_horse = git https://github.com/ninenines/horse.git master
 dep_jsx = git https://github.com/talentdeficit/jsx v2.10.0
 dep_decimal = git https://github.com/egobrain/decimal 0.6.2
-dep_structured-header-tests = git https://github.com/httpwg/structured-header-tests main
+dep_structured-header-tests = git https://github.com/httpwg/structured-header-tests faed1f92942abd4fb5d61b1f9f0dc359f499f1d7
 dep_uritemplate-tests = git https://github.com/uri-templates/uritemplate-test master
 
 # CI configuration.