[lldb-server unittest] Add missing teardown logic

Summary:
This test base class is missing the teardown making the second set of tests extending it to fail in an assertion in the FileSystem::Initialize() (as it's being initialized twice).
Not sure why this isn't failing the build bots.. (unless they're running without asserts?).

With this fix `ninja LLDBServerTests && ./tools/lldb/unittests/tools/lldb-server/tests/LLDBServerTests` successfully runs and passes all tests.

Reviewers: clayborg, xiaobai, labath

Reviewed By: xiaobai, labath

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D62788

llvm-svn: 362406