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>

wgengine/magicsock/magicsock_linux_test.go

@@ -184,19 +184,19 @@ func TestBpfDiscardV4(t *testing.T) {
 		accept  bool
 	}{
 		{
-			name:    "base accepted datagram",
+			name:    "base-accepted-datagram",
 			replace: map[int]byte{},
 			accept:  true,
 		},
 		{
-			name: "more fragments",
+			name: "more-fragments",
 			replace: map[int]byte{
 				6: 0x20,
 			},
 			accept: false,
 		},
 		{
-			name: "some fragment",
+			name: "some-fragment",
 			replace: map[int]byte{
 				7: 0x01,
 			},