Cleanup RNTesterExampleFilter/RNTesterPage/RNTesterBlock Spacers

Summary:
Right now RNTestePage either adds a large spacer to the end of content, unless `noSpacer` is added. But with `noSpacer`, scrolling content may not be reachable because we are putting padding on the wrong container.

This fixes that and removes a few cases where we had multi hundred px empty space in favor of uniformly taking up the parent, and a 10px padding. This also moves the constant margin at the top of RNTesterBlock to the container, so that in the E2E container we can save the 30px.

Changelog:
[Internal]

Reviewed By: javache

Differential Revision: D44197303

fbshipit-source-id: 8dc67f3588bc28316e2fee8d25a0bc59995f1728