[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