test: add missing test flags

test-wasi-start-validation.js should require the
--experimental-wasi-unstable-preview0 flag in order to run.
However, due to a recent regression, that hasn't been enforced.
#30963 fixes the regression
and will cause this test to start (correctly) failing. This
commit adds the missing flag.

PR-URL: #30971
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>

test/wasi/test-wasi-start-validation.js

@@ -1,3 +1,4 @@
+// Flags: --experimental-wasi-unstable-preview0
 'use strict';
 
 require('../common');