[sanitizer] s/TestOnlyInit/Init for the allocator ByteMap (NFC)

Summary:
The `TestOnlyInit` function of `{Flat,TwoLevel}ByteMap` seems to be a misnomer
since the function is used outside of tests as well, namely in
`SizeClassAllocator32::Init`. Rename it to `Init` and update the callers.

Reviewers: alekseyshl, vitalybuka

Reviewed By: vitalybuka

Subscribers: kubamracek, delcypher, #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D46408

llvm-svn: 331662