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/k8s-operator/tsrecorder_specs_test.go

@@ -17,7 +17,7 @@ import (
 )
 
 func TestRecorderSpecs(t *testing.T) {
-	t.Run("ensure spec fields are passed through correctly", func(t *testing.T) {
+	t.Run("spec-fields-passthrough", func(t *testing.T) {
 		tsr := &tsapi.Recorder{
 			ObjectMeta: metav1.ObjectMeta{
 				Name: "test",