Update shim to unbreak OSS tests buck build (#8362)

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

Author: swolchok

shim/BUCK

@@ -1,4 +1,5 @@
 load("@prelude//platforms:defs.bzl", "execution_platform")
+load("@prelude//tests:test_toolchain.bzl", "noop_test_toolchain")
 load("@prelude//toolchains:cxx.bzl", "system_cxx_toolchain")
 load("@prelude//toolchains:genrule.bzl", "system_genrule_toolchain")
 load("@prelude//toolchains:go.bzl", "system_go_toolchain")
@@ -8,6 +9,10 @@ load("@prelude//toolchains:python.bzl", "system_python_bootstrap_toolchain", "sy
 load("@prelude//toolchains:remote_test_execution.bzl", "remote_test_execution_toolchain")
 load("@prelude//toolchains:rust.bzl", "system_rust_toolchain")
 
+# TODO: sync this directory with https://github.com/facebook/buck2-shims-meta.
+# Internal context:
+# https://fb.workplace.com/groups/222849770514616/posts/600883896044533/
+
 oncall("executorch")
 
 system_cxx_toolchain(
@@ -74,3 +79,8 @@ execution_platform(
     use_windows_path_separators = host_info().os.is_windows,
     visibility = ["PUBLIC"],
 )
+
+noop_test_toolchain(
+    name = "test",
+    visibility = ["PUBLIC"],
+)