cmd/vet: add subtestnames analyzer; fix all existing violations

Add a new vet analyzer that checks t.Run subtest names don't contain
characters requiring quoting when re-running via "go test -run". This
enforces the style guide rule: don't use spaces or punctuation in
subtest names.

The analyzer flags:
- Direct t.Run calls with string literal names containing spaces,
  regex metacharacters, quotes, or other problematic characters
- Table-driven t.Run(tt.name, ...) calls where tt ranges over a
  slice/map literal with bad name field values

Also fix all 978 existing violations across 81 test files, replacing
spaces with hyphens and shortening long sentence-like names to concise
hyphenated forms.

Updates #19242

Change-Id: Ib0ad96a111bd8e764582d1d4902fe2599454ab65
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

.github/workflows/vet.yml

@@ -36,8 +36,10 @@ jobs:
 
       - name: Run 'go vet'
         working-directory: src
+        # Use listpkgs --ignore-3p to skip tempfork/ packages, which
+        # intentionally match upstream and may not follow our style rules.
         # Must use ./... instead of tailscale.com/... because the latter will
         # include the v2 go client (tailscale.com/client/tailscale/v2) if it's
         # a dependency in our go.mod file. Possibly a go vet bug, but avoid
         # cross-repo vetting for now so we can safely add the dependency.
-        run: ./tool/go vet -vettool=/tmp/vettool ./...
+        run: ./tool/go vet -vettool=/tmp/vettool $(./tool/go run ./tool/listpkgs --ignore-3p ./...)