[compiler-rt][test] Set shared_libasan_path in test/asan/lit.cfg on Solaris

While checking warnings from the Solaris buildbots, I noticed

  llvm-lit: /opt/llvm-buildbot/home/solaris11-amd64/clang-solaris11-amd64/llvm/projects/compiler-rt/test/asan/lit.cfg:119: warning: %shared_libasan substitution not set but dynamic ASan is available.

Fixed as follows.  Tested on x86_64-pc-solaris2.11.

Differential Revision: https://reviews.llvm.org/D63761

llvm-svn: 364394