Reduce ErrorsFutureStrictReservedWords test to make it faster

Since checking all possible combinations is taking so long this
reduces the test to test the odd cases (let, yield and static) as
well as a single ordinary case.

BUG=v8:3707
LOG=n

Review URL: https://codereview.chromium.org/724713004

Cr-Commit-Position: refs/heads/master@{#25428}