Fixes regression in childViewLayout

Regression introduced by this commit:
https://github.com/sproutcore/sproutcore/commit/ea620753a313f085460de2ba
9eae3c7fe7368d87
Adds unit tests to childViewLayout
Fixes typo in childViewLayout tests
Removes FLEX_GRID tests
Merge pull request #1128 from sproutcore/team/nicolasbadia/child-view-layout-tests

Adds additional unit test for VERTICAL_STACK layout plugin.