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>

net/stun/stun_test.go

@@ -60,7 +60,7 @@ var responseTests = []struct {
 		wantPort: 59029,
 	},
 	{
-		name: "stun.sipgate.net:10000",
+		name: "stun-sipgate-net-10000",
 		data: []byte{
 			0x01, 0x01, 0x00, 0x44, 0x21, 0x12, 0xa4, 0x42,
 			0x48, 0x2e, 0xb6, 0x47, 0x15, 0xe8, 0xb2, 0x8e,
@@ -82,7 +82,7 @@ var responseTests = []struct {
 		wantPort: 58539,
 	},
 	{
-		name: "stun.powervoip.com:3478",
+		name: "stun-powervoip-com-3478",
 		data: []byte{
 			0x01, 0x01, 0x00, 0x24, 0x21, 0x12, 0xa4, 0x42,
 			0x7e, 0x57, 0x96, 0x68, 0x29, 0xf4, 0x44, 0x60,
@@ -100,7 +100,7 @@ var responseTests = []struct {
 		wantPort: 59859,
 	},
 	{
-		name: "in-process pion server",
+		name: "in-process-pion-server",
 		data: []byte{
 			0x01, 0x01, 0x00, 0x24, 0x21, 0x12, 0xa4, 0x42,
 			0xeb, 0xc2, 0xd3, 0x6e, 0xf4, 0x71, 0x21, 0x7c,
@@ -119,7 +119,7 @@ var responseTests = []struct {
 		wantPort: 61300,
 	},
 	{
-		name: "stuntman-server ipv6",
+		name: "stuntman-server-ipv6",
 		data: []byte{
 			0x01, 0x01, 0x00, 0x48, 0x21, 0x12, 0xa4, 0x42,
 			0x06, 0xf5, 0x66, 0x85, 0xd2, 0x8a, 0xf3, 0xe6,