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/vet/vet.go

@@ -9,6 +9,7 @@ import (
 
 	"golang.org/x/tools/go/analysis/unitchecker"
 	"tailscale.com/cmd/vet/jsontags"
+	"tailscale.com/cmd/vet/subtestnames"
 )
 
 //go:embed jsontags_allowlist
@@ -20,5 +21,5 @@ func init() {
 }
 
 func main() {
-	unitchecker.Main(jsontags.Analyzer)
+	unitchecker.Main(jsontags.Analyzer, subtestnames.Analyzer)
 }