Add more tests for SHA256TransactionStore.

One of the tests is a known failure because of a bug, so mark it accordingly.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Silence ResourceWarning in tests.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Properly report open mode to transaction stores.

The mode was always being reported as "w" (write), which resulted in SHA-256
checksums not being validated on load.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>