m/pkg/{logtree,supervisor}: add test helpers

This adds two functions:

  logtree.PipeAllToStderr
  supervisor.NewHarness

These are designed to simplify tests that exercise code which expects to
be run as a supervisor runnable and/or have access to a logtree
instance.

Change-Id: Ibce77aa4927515af7c273d07ced15215ff456ecc
Reviewed-on: https://review.monogon.dev/c/monogon/+/205
Reviewed-by: Leopold Schabel <leo@nexantic.com>
diff --git a/metropolis/pkg/logtree/BUILD.bazel b/metropolis/pkg/logtree/BUILD.bazel
index d325e42..f49430e 100644
--- a/metropolis/pkg/logtree/BUILD.bazel
+++ b/metropolis/pkg/logtree/BUILD.bazel
@@ -14,6 +14,7 @@
         "logtree_access.go",
         "logtree_entry.go",
         "logtree_publisher.go",
+        "testhelpers.go",
     ],
     importpath = "source.monogon.dev/metropolis/pkg/logtree",
     visibility = ["//metropolis:__subpackages__"],