Add lldb-vscode as a dependency of lldb tests.

Summary:
In the current state, 'ninja check-lldb' runs the lldb-vscode tests, but it
won't rebuild lldb-vscode if any of its sources has changed. This is very
confusing when you fix something and the tests keep failing, or vice versa.

Reviewers: clayborg

Subscribers: mgorny, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D59828

llvm-svn: 357016