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>

drive/driveimpl/drive_test.go

@@ -156,27 +156,27 @@ func TestMissingPaths(t *testing.T) {
 		wantStatus int
 	}{
 		{
-			name:       "empty path",
+			name:       "empty-path",
 			path:       "",
 			wantStatus: http.StatusForbidden,
 		},
 		{
-			name:       "single slash",
+			name:       "single-slash",
 			path:       "/",
 			wantStatus: http.StatusForbidden,
 		},
 		{
-			name:       "only token",
+			name:       "only-token",
 			path:       "/" + secretToken,
 			wantStatus: http.StatusBadRequest,
 		},
 		{
-			name:       "token with trailing slash",
+			name:       "token-trailing-slash",
 			path:       "/" + secretToken + "/",
 			wantStatus: http.StatusBadRequest,
 		},
 		{
-			name:       "token and invalid share",
+			name:       "token-invalid-share",
 			path:       "/" + secretToken + "/nonexistentshare",
 			wantStatus: http.StatusNotFound,
 		},