[SYCL][Graph] Add test for linear graph optimizations

[mmichel11]

Follow-up to #20291

• Adds unit test to ensure linear graphs are not tracking sync points and non-linear graphs are tracking sync points.
• Forward declares a test friend class in exec_graph_impl to be able to inspect the optimizations performed on private members.

[mmichel11]

Followed a similar approach to validating class private members that is used in other places, e.g.

llvm/sycl/source/detail/program_manager/program_manager.hpp

Lines 52 to 53 in dfd72fc

	// For testing purposes
	class ProgramManagerTest;

llvm/sycl/source/detail/program_manager/program_manager.hpp

Line 566 in dfd72fc

friend class ::ProgramManagerTest;

[mmichel11]

CI failure is an unrelated known issue: #19951

[reble]

Wondering if we could use something like std::optional<sycl::event> here, to improve the no-event state recording. Since event is only really needed for the out-of-order queue case and its explicit dependencies.

[mmichel11]

I switched to use the eventless single_task enqueue function for the in-order case. The out-of-order case still uses explicit dependencies with events. Also switched to use std::optional<sycl::event>