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>

log/sockstatlog/logger_test.go

@@ -39,19 +39,19 @@ func TestDelta(t *testing.T) {
 		wantStats map[sockstats.Label]deltaStat
 	}{
 		{
-			name:      "nil a stat",
+			name:      "nil-a-stat",
 			a:         nil,
 			b:         &sockstats.SockStats{},
 			wantStats: nil,
 		},
 		{
-			name:      "nil b stat",
+			name:      "nil-b-stat",
 			a:         &sockstats.SockStats{},
 			b:         nil,
 			wantStats: nil,
 		},
 		{
-			name: "no change",
+			name: "no-change",
 			a: &sockstats.SockStats{
 				Stats: map[sockstats.Label]sockstats.SockStat{
 					sockstats.LabelDERPHTTPClient: {
@@ -69,7 +69,7 @@ func TestDelta(t *testing.T) {
 			wantStats: nil,
 		},
 		{
-			name: "tx after empty stat",
+			name: "tx-after-empty-stat",
 			a:    &sockstats.SockStats{},
 			b: &sockstats.SockStats{
 				Stats: map[sockstats.Label]sockstats.SockStat{
@@ -83,7 +83,7 @@ func TestDelta(t *testing.T) {
 			},
 		},
 		{
-			name: "rx after non-empty stat",
+			name: "rx-after-non-empty-stat",
 			a: &sockstats.SockStats{
 				Stats: map[sockstats.Label]sockstats.SockStat{
 					sockstats.LabelDERPHTTPClient: {