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>

control/controlclient/map_test.go

@@ -801,7 +801,7 @@ func TestUpdateDiscoForNodeCallbackWithFullNetmap(t *testing.T) {
 		expectNewDisco  bool
 	}{
 		{
-			name:            "disco key wired through when newer lastSeen",
+			name:            "disco-key-newer-lastSeen",
 			initialOnline:   false,
 			initialLastSeen: oldTime,
 			updateOnline:    false,
@@ -809,7 +809,7 @@ func TestUpdateDiscoForNodeCallbackWithFullNetmap(t *testing.T) {
 			expectNewDisco:  true,
 		},
 		{
-			name:            "disco key NOT wired through when older lastSeen",
+			name:            "disco-key-older-lastSeen",
 			initialOnline:   false,
 			initialLastSeen: now,
 			updateOnline:    false,
@@ -817,7 +817,7 @@ func TestUpdateDiscoForNodeCallbackWithFullNetmap(t *testing.T) {
 			expectNewDisco:  false,
 		},
 		{
-			name:            "disco key wired through when newer lastSeen, going offline",
+			name:            "disco-key-newer-lastSeen-going-offline",
 			initialOnline:   true,
 			initialLastSeen: oldTime,
 			updateOnline:    false,
@@ -825,7 +825,7 @@ func TestUpdateDiscoForNodeCallbackWithFullNetmap(t *testing.T) {
 			expectNewDisco:  true,
 		},
 		{
-			name:            "online flip with newer lastSeen",
+			name:            "online-flip-newer-lastSeen",
 			initialOnline:   false,
 			initialLastSeen: oldTime,
 			updateOnline:    true,