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/netcheck/netcheck_test.go

@@ -998,7 +998,7 @@ func TestNodeAddrResolve(t *testing.T) {
 				}
 				t.Logf("got IPv6 addr: %v", ap)
 			})
-			t.Run("IPv6 Failure", func(t *testing.T) {
+			t.Run("IPv6-Failure", func(t *testing.T) {
 				ap, ok := c.nodeAddrPort(ctx, dnV4Only, dn.STUNPort, probeIPv6)
 				if ok {
 					t.Fatalf("expected no addr but got: %v", ap)