[lldb][unittest] Add call_once flag to initialize debugger (#80786)

I tried adding a new unit test to the core test
suite (https://github.com/llvm/llvm-project/pull/79533) but it broke the
test suite on AArch64 Linux due to hitting an assertion for calling
`Debugger::Initialize` more than once. When the unit test suite is
invoked as a standalone binary the test suite state is shared, and
`Debugger::Initialize` gets called in `DiagnosticEventTest.cpp` before
being called in `ProgressReportTest.cpp`.

`DiagnosticEventTest.cpp` uses a call_once flag to initialize the
debugger but it's local to that test. This commit adds a once_flag to
`TestUtilities` so that `Debugger::Initialize` can be called once by the
tests that use it.