lldb-server tests: Propagate environment variables (pr34192)

Summary:
Without this, the launching of the test inferior may fail if it depends
on some component of the environment (most likely LD_LIBRARY_PATH). This
makes sure we propagate the environment variable to the inferior
process.

Reviewers: eugene

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D39010

llvm-svn: 316244