appium: use snapshots for various schema tests

[boneskull]

there's at least one test in there that looks at flattened-schema.js, which is a test fixture that's supposed to be derived from the actual schema via a flattenSchema() function (the exact name escapes me atm). change this to use a snapshot. see packages/typedoc-plugin-appium/test/e2e/theme/output.e2e.spec.ts for an example of how to do this using mocha-chai-jest-snapshot