[lldb][test][NFC] Create a separate LLDB_TEST_SRC var to allow moving tests.

Summary:
This creates a separate LLDB_TEST_SRC var to match the existing LLDB_TEST var. LLDB_TEST points to the test framework, LLDB_TEST_SRC points to the tests themselves.

The var points to the same place, but a future patch will move the tree + update var.

Reviewers: labath, JDevlieghere

Reviewed By: labath

Subscribers: merge_guards_bot, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D71150