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>

derp/derp_test.go

@@ -616,13 +616,13 @@ func TestSendFreeze(t *testing.T) {
 		}
 	}
 
-	t.Run("initial send", func(t *testing.T) {
+	t.Run("initial-send", func(t *testing.T) {
 		drain(t, "bob")
 		drain(t, "cathy")
 		isEmpty(t, "alice")
 	})
 
-	t.Run("block cathy", func(t *testing.T) {
+	t.Run("block-cathy", func(t *testing.T) {
 		// Block cathy. Now the cathyConn buffer will fill up quickly,
 		// and the derp server will back up.
 		cathyConn.SetReadBlock(true)