snapuserd: Add error propagation to tests.

snapuserd_test in general doesn't propagate errors, which means the
first failure will cascade into many failures. This can make it harder
to figure out what's going on.

Unfortunately error propagation is difficult with gtest, since bool
returns don't work with ASSERT_EQ and Android doesn't use exceptions.

Use ASSERT_NO_FATAL_FAILURE for now, since that is the least invasive
change.

Bug: 288273605
Test: snapuserd_test
Change-Id: I6550682fd63602bc15649f705def2915f6329357