IDL compiler: Working tests for setters (6 extended attributes)

Test-only change, removing |readonly|.
The following extended attributes already work for setters:
[NotEnumerable]
[PerContextEnabled]
[PerWorldBindings]
[RuntimeEnabled]
[TreatReturnedNullStringAs]
[Unforgeable]
This removes |readonly| and asserts that they work for setters.
Batching per request:
https://codereview.chromium.org/31843002/#msg2

Remaining extended attributes do require logic changes.

BUG=239771
NOTRY=true

Review URL: https://codereview.chromium.org/31903002

git-svn-id: svn://svn.chromium.org/blink/trunk@160066 bbb929c8-8fbe-4397-9dbb-9b2b20218538