Preparsing inner funcs: simplify test-parsing/NoPessimisticContextAllocation.

Reduce duplication of the inner function declaration + now we get the
test for inner arrow functions for free (if we later implement lazy
inner arrow functions).

BUG=v8:5501, v8:5516

Review-Url: https://codereview.chromium.org/2637003005
Cr-Commit-Position: refs/heads/master@{#42412}