Some Test262 tests run forever

[Razican]

Describe the bug
We are ignoring some Test262 tests because they run forever. Here you have the list of them:

• test/built-ins/String/prototype/padStart/fill-string-empty.js
• test/built-ins/String/prototype/padEnd/fill-string-empty.js
• test/built-ins/Array/prototype/map/15.4.4.19-8-c-ii-1.js
• test/built-ins/Array/prototype/unshift/throws-if-integer-limit-exceeded.js
• test/built-ins/Array/prototype/concat/arg-length-exceeding-integer-limit.js
• test/built-ins/Array/prototype/includes/length-boundaries.js

To Reproduce
You can reproduce any of them by running the following command after removing them from the test_ignore.txt file:

cargo run --release --bin boa_tester -- run -s {test_path}

Expected behavior
These tests should run reasonably fast (usually not more than a second), but they run forever. A fix should make this happen. Some of them might be easier to fix than others.

[neeldug]

For the concat, I understand there is a check missing from spec here: iii. If n + len > 253 - 1, throw a TypeError exception., however, I'm unable to understand completely how the implementation follows the spec for the spreadable section here. Actually I believe test262 may have an error, or maybe it's my interpretation of the ES standard.