Cleanup IDL test files

Some fixes to IDL test files:
* Move SVG tests to separate TestSVG.idl, as we will not need these in future
(so we can just delete this test case when SVG rewrite complete)

Remove as unused in real IDL files:
* callback interface arguments in constructors
* nullable array

Style fix to TestTypedefs:
* typedefs at *top* of file

BUG=239771
TBR=haraken
NOTRY=true

Review URL: https://codereview.chromium.org/137773002

git-svn-id: svn://svn.chromium.org/blink/trunk@165017 bbb929c8-8fbe-4397-9dbb-9b2b20218538