Check `.gas-snapshot` with `--snapshot-check` flag

[schaable]

Describe the issue

Tasks:

• Add --snapshot-check flag to test and test solidity tasks
• Compare current gas usage with stored .gas-snapshot (unit tests: single value, fuzz tests: median, invariant: skip)
• Return non-zero exit code on mismatch
• Create .gas-snapshot if it doesn't exist (don't fail)
• Handle new tests (add entries without failing)