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>

net/netmon/netmon_linux_test.go

@@ -49,7 +49,7 @@ func TestIgnoreDuplicateNEWADDR(t *testing.T) {
 		return msg
 	}
 
-	t.Run("suppress duplicate NEWADDRs", func(t *testing.T) {
+	t.Run("suppress-duplicate-NEWADDRs", func(t *testing.T) {
 		c := nlConn{
 			buffered: []netlink.Message{
 				newAddrMsg(1, "192.168.0.5", unix.RTM_NEWADDR),
@@ -69,7 +69,7 @@ func TestIgnoreDuplicateNEWADDR(t *testing.T) {
 		}
 	})
 
-	t.Run("do not suppress after DELADDR", func(t *testing.T) {
+	t.Run("no-suppress-after-DELADDR", func(t *testing.T) {
 		c := nlConn{
 			buffered: []netlink.Message{
 				newAddrMsg(1, "192.168.0.5", unix.RTM_NEWADDR),