Fix exit code of validStrings test
Fix exit code of validStrings test

The test should fail with a nonzero exit code if any of the fuzz modes
fails. Otherwise, the CI job won't fail.

UPSTREAM_COMMIT=e88ae8bfa221b703cd1f60e8349c669636f80df1

js-temporal/temporal-polyfill,js-temporal/temporal-polyfill,js-temporal/temporal-polyfill