libsnapshot: Improve test reliability.

The test suite is still quite buggy if interrupted. This fixes a number
of issues (such as bad ordering of setup calls), and refactors things to
add more ASSERTs.

Bug: 139204329
Test: libsnapshot_test gtest
Change-Id: I224608715c29f343b34512a9ac1143f0dde932e9
1 file changed