Use a Nullopt() matcher in mutex_guarded_test
Use a Nullopt() matcher in mutex_guarded_test

This avoids comparing absl::optional<LockView<int>> objects in the test
using operator==.

Also change some ASSERTs to EXPECTs to let the test continue when
expectations fail.

PiperOrigin-RevId: 285499878
Change-Id: I7670365190b93e86fd12a56e5ae41a53c41b5fed

google/asylo,google/asylo,google/asylo,google/asylo,google/asylo,google/asylo