libsnapshot: Improve first test-run and test cleanup.

This CL fixes a bug where libsnapshot_test failed on the first run. It
also fixes bugs where it could not run if it died in the middle of a
test.

Previously, libsnapshot_test relied on CancelUpdate() to perform
cleanup, which cannot run in certain states. Instead, manually delete
dm devices and COW image files, and forcefully erase any lingering data.

Bug: 136678799
Test: libsnapshot_test gtest
Change-Id: I7b2399a403b387eb47184626e71dcf8674f6ab89