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>

cmd/tailscaled/tailscaled_test.go

@@ -58,7 +58,7 @@ func TestStateStoreError(t *testing.T) {
 	args.statedir = t.TempDir()
 	args.tunname = "userspace-networking"
 
-	t.Run("new state", func(t *testing.T) {
+	t.Run("new-state", func(t *testing.T) {
 		sys := tsd.NewSystem()
 		sys.NetMon.Set(must.Get(netmon.New(sys.Bus.Get(), t.Logf)))
 		lb, err := getLocalBackend(t.Context(), t.Logf, logID.Public(), sys)
@@ -70,7 +70,7 @@ func TestStateStoreError(t *testing.T) {
 			t.Errorf("StateStoreHealth is unhealthy on fresh LocalBackend:\n%s", strings.Join(lb.HealthTracker().Strings(), "\n"))
 		}
 	})
-	t.Run("corrupt state", func(t *testing.T) {
+	t.Run("corrupt-state", func(t *testing.T) {
 		sys := tsd.NewSystem()
 		sys.NetMon.Set(must.Get(netmon.New(sys.Bus.Get(), t.Logf)))
 		// Populate the state file with something that will fail to parse to