[lldb/test] Do a better job at setting (DY)LD_LIBRARY_PATH

Summary:
registerSharedLibrariesWithTarget was setting the library path
environment variable to the process build directory, but the function is
also accepting libraries in other directories (in which case they won't
be found automatically).

This patch makes the function set the path variable correctly for these
libraries too. This enables us to remove the code for setting the path
variable in TestWeakSymbols.py, which was working only accidentally --
it was relying on the fact that
  launch_info.SetEnvironmentEntries(..., append=True)
would not overwrite the path variable it has set, but that is going to
change with D83306.

Reviewers: davide, jingham

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D83552