Previously, the snapshot file was recomputed on every build. We now
record checksums for all snapshot inputs (which are catalogued in the
snapshot dependencies file output alongside the snapshot) and only
rebuild if the checksum for any input file has changed.
Relocates two tests alongside other related tests:
* moved code_signing_test.dart alongside other lib/src/ios tests
* moved terminal_test.dart alongside other lib/src/base tests
