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>

wgengine/watchdog_test.go

@@ -26,7 +26,7 @@ func TestWatchdog(t *testing.T) {
 		maxWaitMultiple = 15
 	}
 
-	t.Run("default watchdog does not fire", func(t *testing.T) {
+	t.Run("default-watchdog-does-not-fire", func(t *testing.T) {
 		t.Parallel()
 		bus := eventbustest.NewBus(t)
 		ht := health.NewTracker(bus)
@@ -55,7 +55,7 @@ func TestWatchdogMetrics(t *testing.T) {
 		wantCounts map[watchdogEvent]int64
 	}{
 		{
-			name:   "single event types",
+			name:   "single-event-types",
 			events: []watchdogEvent{RequestStatus, PeerForIPEvent, Ping},
 			wantCounts: map[watchdogEvent]int64{
 				RequestStatus:  1,
@@ -64,7 +64,7 @@ func TestWatchdogMetrics(t *testing.T) {
 			},
 		},
 		{
-			name:   "repeated events",
+			name:   "repeated-events",
 			events: []watchdogEvent{RequestStatus, RequestStatus, Ping, RequestStatus},
 			wantCounts: map[watchdogEvent]int64{
 				RequestStatus: 3,