[ADT] Update SmallVectorTest.EmplaceBack tests after rL356312

rL356312 changed the return type of emplace_back from void to reference.
Update the tests to check the behavior.

Reviewers: dblaikie

Reviewed By: dblaikie

Subscribers: dexonsmith, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59809

llvm-svn: 356980