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>

tailcfg/tailcfg_test.go

@@ -841,12 +841,12 @@ func TestMarshalToRawMessageAndBack(t *testing.T) {
 			capType: PeerCapability("foo"),
 		},
 		{
-			name:    "some values",
+			name:    "some-values",
 			val:     testRule{Ports: []int{80, 443}, Name: "foo"},
 			capType: PeerCapability("foo"),
 		},
 		{
-			name:    "all values",
+			name:    "all-values",
 			val:     testRule{Ports: []int{80, 443}, Name: "foo", ToggleOn: true, Groups: inner{Groups: []string{"foo", "bar"}}, Addrs: []netip.AddrPort{testip}},
 			capType: PeerCapability("foo"),
 		},