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>

util/linuxfw/nftables_runner_test.go

@@ -960,32 +960,32 @@ func TestPickFirewallModeFromInstalledRules(t *testing.T) {
 		want FirewallMode
 	}{
 		{
-			name: "using iptables legacy",
+			name: "using-iptables-legacy",
 			det:  &testFWDetector{iptRuleCount: 1},
 			want: FirewallModeIPTables,
 		},
 		{
-			name: "using nftables",
+			name: "using-nftables",
 			det:  &testFWDetector{nftRuleCount: 1},
 			want: FirewallModeNfTables,
 		},
 		{
-			name: "using both iptables and nftables",
+			name: "using-both-iptables-and-nftables",
 			det:  &testFWDetector{iptRuleCount: 2, nftRuleCount: 2},
 			want: FirewallModeNfTables,
 		},
 		{
-			name: "not using any firewall, both available",
+			name: "no-firewall-both-available",
 			det:  &testFWDetector{},
 			want: FirewallModeNfTables,
 		},
 		{
-			name: "not using any firewall, iptables available only",
+			name: "no-firewall-iptables-only",
 			det:  &testFWDetector{iptRuleCount: 1, nftErr: errors.New("nft error")},
 			want: FirewallModeIPTables,
 		},
 		{
-			name: "not using any firewall, nftables available only",
+			name: "no-firewall-nftables-only",
 			det:  &testFWDetector{iptErr: errors.New("iptables error"), nftRuleCount: 1},
 			want: FirewallModeNfTables,
 		},