flaky test for ASIO completion tokens

[ericniebler]

as noted here, one of the completion token tests is flaky and needs to be investigated.

stdexec/test/exec/asio/test_completion_token.cpp

Lines 354 to 366 in 849f989

	// This function is seemingly fragile?
	//
	// In release mode slightly different arrangements produce bizarre outcomes:
	//
	// /root/stdexec/test/asioexec/test_completion_token.cpp:380: FAILED:
	// CHECK( !ex )
	// with expansion:
	// false
	//
	// /root/stdexec/test/asioexec/test_completion_token.cpp:382: FAILED:
	// CHECK( ptr.use_count() == 1U )
	// with expansion:
	// -1564306560 == 1