[lldb-server unittest] Add missing teardown logic
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
Differential Revision: https://reviews.llvm.org/D62788