Fix path issues in debugger unittests
Summary:
Debugees and their source files were moved to common folder, but the path finding routines in LLDB/GDB weren't changed.
This patch updates the LLDB testhelpers to handle moved debuggees and shares that between LLDB and GDB, along with a few
other improvements in path handling in unittests.
Test Plan: most LLDB/GDB unit tests pass after the patch, which fail without the patch.
Reviewers: apol, KDevelop, kfunk
Reviewed By: KDevelop, kfunk
Subscribers: kfunk, kdevelop-devel
Differential Revision: https://phabricator.kde.org/D6887