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/tshttpproxy/tshttpproxy_test.go

@@ -97,7 +97,7 @@ func TestSetSelfProxy(t *testing.T) {
 		wantHTTPS string
 	}{
 		{
-			name: "no self proxy",
+			name: "no-self-proxy",
 			env: map[string]string{
 				"HTTP_PROXY":  "127.0.0.1:1234",
 				"HTTPS_PROXY": "127.0.0.1:1234",
@@ -107,7 +107,7 @@ func TestSetSelfProxy(t *testing.T) {
 			wantHTTPS: "127.0.0.1:1234",
 		},
 		{
-			name: "skip proxies",
+			name: "skip-proxies",
 			env: map[string]string{
 				"HTTP_PROXY":  "127.0.0.1:1234",
 				"HTTPS_PROXY": "127.0.0.1:5678",
@@ -117,7 +117,7 @@ func TestSetSelfProxy(t *testing.T) {
 			wantHTTPS: "", // skipped
 		},
 		{
-			name: "localhost normalization of env var",
+			name: "localhost-normalization-of-env-var",
 			env: map[string]string{
 				"HTTP_PROXY":  "localhost:1234",
 				"HTTPS_PROXY": "[::1]:5678",
@@ -127,7 +127,7 @@ func TestSetSelfProxy(t *testing.T) {
 			wantHTTPS: "", // skipped
 		},
 		{
-			name: "localhost normalization of addr",
+			name: "localhost-normalization-of-addr",
 			env: map[string]string{
 				"HTTP_PROXY":  "127.0.0.1:1234",
 				"HTTPS_PROXY": "127.0.0.1:1234",
@@ -137,7 +137,7 @@ func TestSetSelfProxy(t *testing.T) {
 			wantHTTPS: "", // skipped
 		},
 		{
-			name: "no ports",
+			name: "no-ports",
 			env: map[string]string{
 				"HTTP_PROXY":  "myproxy",
 				"HTTPS_PROXY": "myproxy",