debuginfo: Gate all LLDB debuginfo tests on a minimum LLDB version being available
debuginfo: Make some path LLDB-related paths in the test runner absolute to help the build bots.
debuginfo: Enable LLDB test suite on Darwin.
debuginfo: Let LLDB tests run in parallel again since our min-supported version has no problems with that.
auto merge of #18092 : michaelwoerister/rust/lldb-test-versioning, r=alexcrichton

Now that there are build bots with a stable enough LLDB version on OSX we can finally let the tests run on every PR!
:joy_cat: