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>

cmd/gitops-pusher/gitops-pusher_test.go

@@ -30,7 +30,7 @@ func TestEmbeddedTypeUnmarshal(t *testing.T) {
 		},
 	}
 
-	t.Run("unmarshal gitops type from acl type", func(t *testing.T) {
+	t.Run("unmarshal-gitops-from-acl", func(t *testing.T) {
 		b, _ := json.Marshal(aclTestErr)
 		var e ACLGitopsTestError
 		err := json.Unmarshal(b, &e)
@@ -41,7 +41,7 @@ func TestEmbeddedTypeUnmarshal(t *testing.T) {
 			t.Fatalf("user heading for 'ACLError' not found in gitops error: %v", e.Error())
 		}
 	})
-	t.Run("unmarshal acl type from gitops type", func(t *testing.T) {
+	t.Run("unmarshal-acl-from-gitops", func(t *testing.T) {
 		b, _ := json.Marshal(gitopsErr)
 		var e tailscale.ACLTestError
 		err := json.Unmarshal(b, &e)