test: increase coverage for assertion_error.js

Add a test for long strings and assert.notDeepEqual() to cover code that truncates output when it is longer than 1024 characters.

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.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines