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>

tstest/integration/vms/vms_test.go

@@ -355,7 +355,7 @@ func (h *Harness) testDistro(t *testing.T, d Distro, ipm ipMapping) {
 		})
 	})
 
-	t.Run("tailscale status", func(t *testing.T) {
+	t.Run("tailscale-status", func(t *testing.T) {
 		dur := 100 * time.Millisecond
 		var outp []byte
 		var err error
@@ -383,7 +383,7 @@ func (h *Harness) testDistro(t *testing.T, d Distro, ipm ipMapping) {
 		t.Fatalf("error: %v", err)
 	})
 
-	t.Run("dump routes", func(t *testing.T) {
+	t.Run("dump-routes", func(t *testing.T) {
 		sess, err := cli.NewSession()
 		if err != nil {
 			t.Fatal(err)