tstest: add the start of a testing wishlist

Of tests we wish we could easily add. One day.

Updates #13038

Change-Id: If44646f8d477674bbf2c9a6e58c3cd8f94a4e8df
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tstest/test-wishlist.md

@@ -0,0 +1,17 @@
+# Testing wishlist
+
+This is a list of tests we'd like to add one day, as our e2e/natlab/VM
+testing infrastructure matures.
+
+We're going to start collecting ideas as we develop PRs (updating this
+wishlist in the same PR that adds something that could be better
+tested) and then use this list to inform the order we build out our
+future testing machinery.
+
+For each item, try to include a `#nnn` or `tailscale/corp#nnn`
+reference to an issue or PR about the feature.
+
+# The list
+
+- ...
+