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>

net/speedtest/speedtest_test.go

@@ -47,7 +47,7 @@ func TestDownload(t *testing.T) {
 	// ensure that the test returns an appropriate number of Result structs
 	expectedLen := int(DefaultDuration.Seconds()) + 1
 
-	t.Run("download test", func(t *testing.T) {
+	t.Run("download-test", func(t *testing.T) {
 		// conduct a download test
 		results, err := RunClient(Download, DefaultDuration, serverIP)
 
@@ -65,7 +65,7 @@ func TestDownload(t *testing.T) {
 		}
 	})
 
-	t.Run("upload test", func(t *testing.T) {
+	t.Run("upload-test", func(t *testing.T) {
 		// conduct an upload test
 		results, err := RunClient(Upload, DefaultDuration, serverIP)