test: add missing test flags

[cjihrig]

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.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines