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>

ipn/ipnlocal/serve_unix_test.go

@@ -23,26 +23,30 @@ import (
 
 func TestExpandProxyArgUnix(t *testing.T) {
 	tests := []struct {
+		name         string
 		input        string
 		wantURL      string
 		wantInsecure bool
 	}{
 		{
+			name:    "unix-tmp-sock",
 			input:   "unix:/tmp/test.sock",
 			wantURL: "unix:/tmp/test.sock",
 		},
 		{
+			name:    "unix-var-run-docker-sock",
 			input:   "unix:/var/run/docker.sock",
 			wantURL: "unix:/var/run/docker.sock",
 		},
 		{
+			name:    "unix-relative-sock",
 			input:   "unix:./relative.sock",
 			wantURL: "unix:./relative.sock",
 		},
 	}
 
 	for _, tt := range tests {
-		t.Run(tt.input, func(t *testing.T) {
+		t.Run(tt.name, func(t *testing.T) {
 			gotURL, gotInsecure := expandProxyArg(tt.input)
 			if gotURL != tt.wantURL {
 				t.Errorf("expandProxyArg(%q) url = %q, want %q", tt.input, gotURL, tt.wantURL)