Update shim to unbreak OSS tests buck build

Porting facebook/buck2-shims-meta@fe8b492 as a short-term measure until I figure out how to get us synced with that repo.

Test Plan: buck2 build //runtime/test/... no longer fails with missing shim//:test error

ghstack-source-id: 3beacda
ghstack-comment-id: 2649766301
Pull Request resolved: #8362

Author: swolchok

shim/BUCK

@@ -1,3 +1,4 @@
+load("@prelude//tests:test_toolchain.bzl", "noop_test_toolchain")
 load("@prelude//platforms:defs.bzl", "execution_platform")
 load("@prelude//toolchains:cxx.bzl", "system_cxx_toolchain")
 load("@prelude//toolchains:genrule.bzl", "system_genrule_toolchain")
@@ -74,3 +75,8 @@ execution_platform(
     use_windows_path_separators = host_info().os.is_windows,
     visibility = ["PUBLIC"],
 )
+
+noop_test_toolchain(
+    name = "test",
+    visibility = ["PUBLIC"],
+)