test: increase coverage for assertion_error.js #26065
Add a test for long strings and assert.notDeepEqual() to cover code that
This introduces coverage for https://coverage.nodejs.org/coverage-0a4c69a5c5e8ba93/lib/internal/assert/assertion_error.js.html#L370.
I standardized nearby arrow functions to use braces when they are not intended to return a value.
Edited the commit message and force-pushed. CI: https://ci.nodejs.org/job/node-test-pull-request/20757/