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>

syncs/shardedint_test.go

@@ -30,7 +30,7 @@ func BenchmarkShardedInt(b *testing.B) {
 		})
 	})
 
-	b.Run("sharded int", func(b *testing.B) {
+	b.Run("sharded-int", func(b *testing.B) {
 		m := NewShardedInt()
 		b.RunParallel(func(pb *testing.PB) {
 			for pb.Next() {
@@ -60,7 +60,7 @@ func TestShardedInt(t *testing.T) {
 		}
 	})
 
-	t.Run("high concurrency", func(t *testing.T) {
+	t.Run("high-concurrency", func(t *testing.T) {
 		m := NewShardedInt()
 		wg := sync.WaitGroup{}
 		numWorkers := 1000
@@ -83,7 +83,7 @@ func TestShardedInt(t *testing.T) {
 		}
 	})
 
-	t.Run("encoding.TextAppender", func(t *testing.T) {
+	t.Run("encoding-TextAppender", func(t *testing.T) {
 		m := NewShardedInt()
 		m.Add(1)
 		b := make([]byte, 0, 10)