[test] Move llvm-lib tests into tools/llvm-lib. NFC.

Similarly to SVN r317189 for llvm-dlltool, these are probably
easier to find in a tools subdirectory with a name identical to
the tool, than in a toplevel directory with a different name.

This matches the move of LibDriver itself in SVN r302995.

Differential Revision: https://reviews.llvm.org/D39531

llvm-svn: 317262