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>

client/tailscale/tailscale_test.go

@@ -31,7 +31,7 @@ func TestClientBuildURL(t *testing.T) {
 			want:     `http://127.0.0.1:1234/api/v2/tailnet/example%20dot%20com%3Ffoo=bar`,
 		},
 		{
-			desc:     "url.Values",
+			desc:     "url-Values",
 			elements: []any{"tailnet", "example.com", "acl", url.Values{"details": {"1"}}},
 			want:     `http://127.0.0.1:1234/api/v2/tailnet/example.com/acl?details=1`,
 		},
@@ -71,7 +71,7 @@ func TestClientBuildTailnetURL(t *testing.T) {
 			want:     `http://127.0.0.1:1234/api/v2/tailnet/example.com/foo%20bar%3Fbaz=qux`,
 		},
 		{
-			desc:     "url.Values",
+			desc:     "url-Values",
 			elements: []any{"acl", url.Values{"details": {"1"}}},
 			want:     `http://127.0.0.1:1234/api/v2/tailnet/example.com/acl?details=1`,
 		},